Like, it really doesn't interest me. There's a reason I only like Tomarrymort when there's a push and pull and Harry and Tom are portrayed as the equals they are. Like, I don't like Tomarrymort where Harry is completely submissive to Voldemort and Bellamort for the same reason — these aren't the kind of relationships that make Tom interesting.

I like both Tom and Bellatrix a lot as individuals, but I don't think a romantic and/or sexual relationship between them pushes their characters to interesting places. They are both stagnate in this relationship and, for me personally, that just doesn't interest me.

Like, Bellatrix is completely submissive to Voldemort nodding her head excitedly and panting after him: "Yes my lord! Whatever you say, my lord!" And this is not the type of dynamic that'd push either character towards growth. They don't push each other into a character arc, which is what I usually like my ships to do.

Additionally, this dynamic basically means Voldemort always gets what he wants, and Bellatrix is happy with it, as you said, she'd shape herself for his every whim. This isn't a relationship between equals. It's a relationship where she worships him and he doesn't respect her or care about her as a person. Like, at all.

The dynamic we see from them in the books gave me the impression Voldemort cares about Bellatrix. He doesn't want her to be hurt or to die:

Bellatrix’s gloating smile froze, her eyes began to bulge: For the tiniest space of time she knew what had happened, and then she toppled, and the watching crowd roared, and Voldemort screamed.

(DH)

But he cares about her like how you care about your favorite pet. He relished in giving her orders and having her submit completely:

“Master, I am sorry, I knew not, I was fighting the Animagus Black!” sobbed Bellatrix, flinging herself down at Voldemort’s feet as he paced slowly nearer. “Master, you should know —” “Be quiet, Bella,” said Voldemort dangerously. “I shall deal with you in a moment. Do you think I have entered the Ministry of Magic to hear your sniveling apologies?” “But Master — he is here — he is below —” Voldemort paid no attention.

(OotP)

He doesn't actually care about her being hurt if it's not too bad, he doesn't care about her feelings or apologies, especially not when Harry is right in front of him — his obsession, his one failure. Bellatrix takes a backseat, basically always. He doesn't care about her all that much. He cares and respects her like a loyal dog, not like a person he has a relationship with.

He also relished in humiliating and embarrassing her. He likes making fun of her in ways Bella clearly does not enjoy, which isn't something you'd do to someone you love:

“I’m talking about your niece, Bellatrix. And your, Lucius and Narcissa. She has just married the werewolf, Remus Lupin. You must be so proud.” There was an eruption of jeering laughter from around the table. Many leaned forward to exchange gleeful looks, a few thumped the table with their fists. The great snake, disliking the disturbance, opened its mouth and hissed angrily, but the Death Eaters did not hear it, so jubilant where that at Bellatrix and the Malfoys’ humiliation. Bellatrix’s face, so recently flushed with happiness, had turned an ugly, blotchy red.

(DH)

she's desperate to please him, to tell him everything she thinks he wants to hear and she happily lets him treat her like fucking dirt. I don't find a relationship like that compelling, as I said, Voldemort would never change for Bellatrix and Bellatrix honestly deserves better than this. He even lets other Death Eaters jeer and laugh at her, this is not a romantic relationship.

Like even if he had sex with her, it was purely physical as he just doesn't care about her as a person like this. As more than a faithful servant (which he enjoys making fun of, as he does so for many of them).

And he is unwilling to show her real, unintentional weakness or ask her for help:

“My Lord, let me—” “I do not require assistance,” said Voldemort coldly, and though he could not see it, Harry pictured Bellatrix withdrawing a helpful hand.

(DH)

He does trust her with one of his Horcrux as the cup is kept in her vault and she seems to know what it is:

“Be quiet! The situation is graver than you can possibly imagine, Cissy! We have a very serious problem!” She stood, panting slightly, looking down at the sword, examining its hilt. Then she turned to look at the silent prisoners. “If it is indeed Potter, he must not be harmed,” she muttered, more to herself than to the others. “The Dark Lord wishes to dispose of Potter himself. . . . But if he finds out . . . I must . . . I must know. . . .”

(DH)

He trusts her loyalty, and she is one of his preferred Death Eaters (he doesn't hate her like he does Wormtail, Tom appreciates courage and loyalty, which are both traits Bellatrix possesses) but he clearly doesn't trust her with his backstory in the first war:

“Shut your mouth!” Bellatrix shrieked. “You dare speak his name with your unworthy lips, you dare besmirch it with your half-blood’s tongue, you dare —” “Did you know he’s a half-blood too?” said Harry recklessly. Hermione gave a little moan in his ear. “Voldemort? Yeah, his mother was a witch but his dad was a Muggle — or has he been telling you lot he’s pureblood?” “STUPEF —” “NO!” A jet of red light had shot from the end of Bellatrix Lestrange’s wand, but Malfoy had deflected it. His spell caused hers to hit the shelf a foot to the left of Harry and several of the glass orbs there shattered. [...] “He dared — he dares —” shrieked Bellatrix incoherently. “— He stands there — filthy half-blood —”

(OotP)

She doesn't really know who Voldemort is. She worships the persona of Voldemort. She loves his lies and masks. She doesn't actually know Tom Riddle. And I don't think she could accept and love the real Tom Riddle behind the title of Voldemort — the poor but brilliant nerdy half-blood who craves recognition. She would find him pathetic.

It's basically Hinny, isn't it?

She adores his persona and fame and what people think he is without actually knowing or understanding him. She changes her personality to fit what she thinks his girl needs to be because she is so focused on being with him. And He likes that she doesn't get in his way and lets him do and say whatever without crying about it but doesn't care about her or her feelings nearly as much as people think.

Bellamort is just Hinny with a different skin, and I never liked Hinny.

Like Hinny, they don't know or understand each other, and it's clear Bella and Ginny care about Voldemort and Harry more than the boys care about them. Like, yes, Harry would be devastated if Ginny died, but he'd get over it way faster than he did about Sirius. Same for Voldemort, he cares about Bella, but not as an equal he understands and cares for the feelings of. Voldemort got over Bella's death fairly quickly as well, he's way more focused on Harry.

So, with all of this, who do I think is the best pairing for Voldemort?

If we're talking about canon characters who are actually characters in the books? Then Harry is my top choice. Harry is the only one Voldemort would see as an equal and can actually push and change Voldemort as much as Voldemort changes him. There is no other character in canon, I believe, who would be able to do this to the level Harry could. Their dynamic is just so mutually obsessive and tense that a relationship like that can't not change both of them in a myriad of interesting ways.

Though, I was thinking about it, and Severus/Voldemort have potential. Voldemort clearly respects Sev and his opinions more than the average Death Eater:

Snape did not speak. “Perhaps you already know it? You are a clever man, after all, Severus. You have been a good and faithful servant, and I regret what must happen.” “My Lord—”

(DH)

He cares about him and regrets having to kill him:

Harry saw Snape’s face losing the little color it had left; it whitened as his black eyes widened, as the snake’s fangs pierced his neck, as he failed to push the enchanted cage off himself, as his knees gave way and he fell to the floor. “I regret it,” said Voldemort coldly

(DH)

Voldy is willing to forgive Sev for things he'd kill most for. They have so much shared experience (poor, muggle childhood in incredibly abusive environments) that would allow them to understand each other. They probably both get frustrated over pureblood idiocy. Both are intelligent and share many interests, like they're both magic nerds who'd talk all night about magical theory...

So, I think, under the right circumstances, Severus is a pretty good pairing for Voldemort.

The only real downside is that depending on when they get together, they'd push each other to be more extremist and overall worse. Like, they'd push each other to have less empathy for other people if they get together, say, during the first war. Well, it might not be a downside. It really depends on how you look at it.

If they get together in the second war, it's different, and in my opinion, more compelling and interesting for both of them. Like, pairing them up after Voldemort's return and after Sev already turned traitor opens so many interesting avenues. I mean, Sev was someone Voldemort actually regretted killing, that was remorse there, wasn't it? It means Severus could push Voldemort to change in a way Bellatrix doesn't. Because Voldemort respects Snape in a way he doesn't respect Bella. I mean, think about how many times Voldemort shut Bella down when she kept insisting Snape is a traitor — it's clear he values Snape more than he values her.

If we're also looking at side characters we don't know as much about, then we have some more options.

@iamnmbr3 has convinced me that Alphard Black/Tom Riddle is an option, and I have been very compelled by it. We don't know much about Alphard, but that never really stopped me because what we do know is interesting.

We know he is Sirius' uncle. He was born after Walburga but before Cygnus, probably closer in age to Walburga. So, I headcanon he was born in 1927 and was in the same year as Tom Riddle.

We know Alphard was a Slytherin since Sirius mentions all his family was in Slytherin, which would include his uncle. And we know Alphard was burned off the family tapestry when he gave Sirius money when Sirius ran away from home.

This leaves us with a character, who's cunning, capable of listening to his older sister Walburga go off about whatever without making the fight worse but has a spine to stand up to her bullshit when it's actually important. This gives him the right characteristics to be able to wrangle a character arc out of a romance with a younger Tom Riddle (and perhaps the older one, too).

He's a pureblood who's open-minded enough to support Sirius and not hate muggleborns (probably). He likely has the subtlety necessary to fix Tom without Tom feeling like he's being fixed. Alphard, used to his very eventful family, is an expert in dealing with dramatic people (like his siblings) and how to undercut their drama instead of pushing them further into their position (which is what Harry would do, for example. Harry and Tom would keep pushing at each other while someone like Alphard would be able to just remove the heat from the argument and allow it to not get as extreme).

Again, it's not much to go on, but it has so much potential.

(Also, @iamnmbr3 has this post about how Voldemort’s violence became worse in 1979, which happens to be the same year Alphard Black died, and while I don't agree with all the points made there, I find it to be a super fun concept)

Voldemort/Lily also has potential. She's smart, stubborn, academically inclined, and has the right rough edges to have the kind of push-and-pull dynamic with Voldemort that I like with Tomarrymort. Lily is probably the kind of witch Voldemort could grow to respect as well. I don't think he would've agreed to spare her for Snape if he didn't respect both of them. JKR also said he tried to recruit James and Lily, so, he was aware that she was talented.

I think, though, Lily/Voldemort would be slightly better than Harry/Voldemort in some aspects. Lily isn't as hot-headed as Harry. Lily's anger is usually much colder, which I feel would work better with Tom just because she wouldn't push all his buttons (just most of them). She would still push him into a character arc, but it would be a gentler nudge than if Harry did it.