Beelieve in Yourself

disregarding the "obvious-shitpost" candidates, so far the presented options that different parts of the fandom are pushing forth are Carol, December, Rudy, Gaster, and an amalgamated fusion of Kris and Noelle. let's cover each of these separately, shall we?

W.D. Gaster

Gaster is never mentioned by name even once in Deltarune, not even in the files. The "fact" that he even exists at all in Deltarune is still, technically, all theoretical, even if the evidence itself is compelling. He cannot be the Knight because as of now, he does not exist and has no ties to any of the major plot points. Could he fill some other role in the future, perhaps? Sure, maybe, but at the moment it still seems unlikely he will show up at all, so don't cross your fingers. Some mysteries are at their best when left as such.

Papyrus

Papyrus is not a character in Deltarune as of now. Yes, there are a lot of random details in the story that reference him, but that's really all they are-- references. Papyrus's name is never mentioned once in the text of Deltarune, and all indirect references to him are extremely missable and not at all required for the main story. While Undertale characters are present in Deltarune's narrative, it has become very clear that most of them are strictly side characters and will not be given the kind of story relevance the fandom expects from them. Deltarune's narrative MUST be understandable by someone who has never played Undertale to be truly separate from it (as Toby clearly intends). Having the MAIN ANTAGONIST actually be a character that shows up out of basically nowhere and has no relevance to the Dreemurrs, the Holidays, or even Hometown itself would make all of the narrative's setup fall completely flat and just confuse the hell out of everyone. Sorry, Papyrus fans, but this just isn't going to happen. He's busy.

Rudy Holiday

Rudy is actually a relevant story character in some respects, so this sounds plausible on the surface. He's got the antlers, the connection to both the Dreemurrs and the Holidays, he's got some possible motives, and there is a bit of evidence that could point in his direction if you squint a bit. However, no matter how compelling this theory can be at times, it shatters because of one thing--

interacting with Rudy at all is completely optional.

To explain why this is important, we have to think about Deltarune as a whole; as a videogame with a linear narrative. The nature of Deltarune being a videogame means that every player will play the game differently. Some will seek out all of the optional interactions and boss fights and secret lore, but some just... won't. Some might blitz through the game without stopping to talk to anyone or do anything that they don't absolutely have to. In order for Deltarune to be successful in telling the story it wants to tell, it must be able to give that kind of player the information they need to-- at the very least-- get the gist of what is happening and where the story is headed. So far, the only moment Rudy has in the narrative that the player is required to witness is the phone call Noelle gets from Carol about him "falling down" in Chapter 5. This sets him up as a point of conflict among the Holiday family and within Noelle as a character, but does NOT do any work towards establishing him as a character beyond that. All other scenes with Rudy Holiday can be completely avoided by a. never visiting the hospital, b. not talking to him or Toriel at the church, and c. not talking to him as he's setting up lights for the festival. If Rudy Holiday truly was, again, the MAIN ANTAGONIST of Deltarune, hiding all of this foreshadowing and character behind easily skippable dialogue would be doing him a massive disservice and overall end up weakening the plot. And I believe that Toby is a better writer than that. (*cough* *cough* also half the reason people started considering him in the first place is because they didn't like that the two most likely candidates are women *cough* *cough* misogyny in the UTDR fandom is still a massive fucking issue *cough*)

Kris & Noelle amalgamation

This one became popular after Chapter 5's Weird Route, mostly because it would be sick as hell and also connect with the "all of this has happened before, it's all an infinitely-repeating cycle" type of theories. While this does include characters that are VERY relevant to the story (I mean, they're the goddamn protagonists), it runs into a similar problem that the Rudy Knight theory does-- it relies far too much on optional content. The Weird Route/Side B is very clearly shown via Ralsei's reactions to it as very much NOT the intended path (the forbidden path, even), and is an obscure part of the game with a convoluted set of requirements that not every player of Deltarune will end up seeing. Thus, having the Knight's identity be reliant on the Weird Route to truly understand would be a very strange choice on Toby's part, one that doesn't seem worth the plot holes and narrative pitfalls that must be navigated around to make such a twist work. Still more likely of a candidate than Rudy, oddly enough, but not one that I see as solid enough to really work well as an answer.

Carol Holiday

Carol, in my opinion, is the character that Deltarune currently wants you to think is the Knight at this point in the story. Her clearly being the voice that Kris is calling on the phone, her naturally imposing and serious presence starkly contrasting the lighthearted goofiness of pretty much everyone else we've met so far, her ominous line about how "YOU are always welcome here", and the various things she is very suspiciously involved in (funding Asgore's conspiracies, investigating by the shelter in Chapter 5, having one of the codes to the shelter in her house); it's all signals that are clearly screaming "HI YES I AM THE ANTAGONIST PAY ATTENTION TO ME". Unlike many of the other Knight candidates, her most relevant scene is actually mandatory to witness (at least when not on the Weird Route that, as I've mentioned, is an intentional deviation from Deltarune's main narrative). However, these are all attributes that applied to Kris as well during Chapters 1 and 2, and plenty of people were 100% sure they were the Knight until Chapter 3 revealed that they were, in fact, not the Knight, but still working with the Knight. So yes, while at this point Carol seems to be the most obvious pick for the Knight's identity, I would not be surprised if Tricky Tony pulls a double red-herring on us and puts Carol in the same boat as Kris. In my eyes its pretty much a 50/50 coin flip as to whether or not this one will happen.

and finally...

December Holiday

So, let's get one thing out of the way-- yes, Dess has yet to physically appear as a character in Deltarune. This is because her role in the plot requires her, at least for the time being, to not be in it. She is the driving force haunting almost every aspect of this narrative, and I will maintain that she is INCREDIBLY relevant to the overarching story of this game. Because, if she wasn't... then why the hell is she mentioned SO GODDAMN OFTEN?

Put aside all the ARG stuff, that's not even super important here. In Deltarune so far, we have her mentioned by name twice by Noelle in mandatory scenes when she was never introduced as a character before this, clearly inviting you to be curious about her. The word "December" then once again is brought up in an unskippable cutscene with Berdly, implying that Noelle lost the spelling bee because she couldn't bear to say her sister's name. Then is Chapter 3 TENNA of all people brings up Dess out of literally nowhere as a way to try and convince Kris to go along with the music challenge, clearly showing that she and Kris had some kind of close relationship. This is confirmed to extend to the rest of the Dreemurrs as well during Tenna's ending monologue cutscene, where in the visuals Dess is shown to disappear first, with the rest of the two families fading out one by one only after she is gone. In Chapter 4, the code to the shelter that we need is hidden inside of December's guitar, meaning we have to enter her empty room to go find it. Carol's icy character introduction is ALSO reliant upon Dess's guitar, with Susie holding it being the main reason why she gets so pissed with her. Then in Chapter 5, the cutscene with Asgore describing his guilt-ridden quest to figure out the truth behind "what happened that day" is also heavily implied to relate to Dess no longer being around. The vast majority of these scenes are required viewing in order to progress the game, and all of these little mentions and connections and sprinkles of information are all clearly pointing the player towards one major question--

what the hell happened to December Holiday?

given that the OTHER big questions of the narrative right now are "who is the Knight?", "what are they planning?", and "what did that last part of the prophecy say?", I would be absolutely SHOCKED if these things do not end up connecting together at the end. Dess either is the Knight or is the primary motivator behind the Knight's actions-- nothing else could justify the amount of focus and narrative weight that the story of Deltarune is putting on her.