Sorry, (former-)fellow Miku fans, but I'm backing Yotsuba now. I remove any partial support I have for Nino, too. I still like M & N, but I just can't see them being the winner anymore.
After reading up on some of the points of the discussion that /a/non teased, and thinking over a few instances not covered in said discussion, it is clearly most likely Yotsuba. Negi-sensei wouldn't have put those instances in if they didn't mean anything.
As much as Itsuki is likely to make that facial expression at the conclusion of the Scrambled Eggs arc, she isn't willing enough to have preformed the preceding kiss, nor is she developed in that way to do so. To break it down, Ichika and Nino are basically running their own corners. Yotsuba is backing Miku, and now we see that Itsuki is backing Yotsuba. Combining that with how things were between Fuutarou and Ituski in the early chapters and arcs of the story, as well as Negi-sensei's admission, there is zero chance that it'd ever be Itsuki.
And why would Yotsuba make such a haunted expression to go with that denial in Chapter 72 if she didn't long for it? There is so much more to her than the surface that we see, plus depending on the timeframe (I'm not going out of my way as I write this in the moment to doublecheck the timeline, busy), it's likely her meeting with Fuutarou in Kyoto made her want to be different to her sisters (and we know how that ended).