Hot take: Buffy is not Angel's true love.
So, I saw a fanfic refer to Buffy as the "love of his life," and, no. Just, no.
I'm not even talking right now about how predatory and abusive their relationship was on his part, though we can certainly talk about that and I have before, at length.
Angel was involved with her for all of two-three years. Out of his centuries-long life. Ending so badly they're barely on speaking terms, when its literally life and death.
But do you know who was there through all (well, most) of those long centuries?
Perhaps its inconsistent characterization, but it is notable that someone as callous and nihilistic as Angelus, someone supernaturally-certified by a judging demon (literally called "the Judge") to have "nothing human" in him, stayed with Darla for centuries. And someone as cynical as Darla stayed with him. Even when he got a soul, and remorse for what he'd done, he tried to go back to her. And even though he wasn't her Angelus any more, she still went to considerable lengths to get him back. And this isn't all retcons from Angel's spin-off series either- the seeds of this relationship were very clearly planted way back in Buffy's first season.
She made him a monster, and they did monstrous things together and eventually betrayed each other- but as toxic and fucked up as it was and as much as they ABSOLUTELY made each other worse (which is impressive, considering where they started out)... those two did absolutely care about one another. For centuries.
And then they both experienced regaining a soul, having to live with what they'd done, dying and being reborn again, being the only vampires known EVER to have a child together...
They are literally the only beings in their universe who could ever really understand what the other has been through.
Darla nailed it when she said she and Angel were "soul mates".... with all the bitterness that that term deserved.
(I remember a post about "Gargoyles" once that referred to Macbeth and Demona as "Soulmates (derogatory)" and I feel that's almost the perfect description of Angel and Darla's relationship).