So we all know that Miku's the best girl, for a lot of reasons. Nino literally drugged some random guy and deserves to be arrested, not a romcom heroine. Being guarded and overly defensive is a fine story arc, and her transition to being the first to openly court MC was fine and even needed to move the series along, but had a huge stark tonal contrast to the rest of the girls that it's hard to get over. Especially because Ichika is going around doing bad shit un-apologetically shitting on her family and basically being Nino Light. Ichika literally calls this out, asking "what's the difference between you and I? The difference is that Ichika started as self sacrificing before getting convinced she can put herself first, and that acted as the impetus for her to start acting as an asshole. Nino started as a bad person and stays a bad person. Actually Nino goes around doing what she wants, and is probably why Nino shitposters actually like her but I'm dismissing it because Miku is best girl. Yotsuba has the mental age of a child and is strictly friend tier, which is good because even in our final destination no items fox only showdown she isn't trying to grab the madcats controller and become player 5. Itsuki is just the fat one, doesn't do much interesting but might win because she was Rena and that was the only way to argue she wasn't ACTUALLY first girl.
Miku was dismissive of USG but was quirky with her preference and could be won over organically. Not only is she the cutest and her distinguishing factor the most distinct, but she's an underdog and you're supposed to root for them, baka (that's japanese for stupid, you baka). Honestly seeing the future marriage scenes and knowing it's Miku has so much more payoff than any of the other girls. If it's Yotsuba it means she stopped being genki during her wedding day, Itsuki means she stopped eating to fit in her dress, Nino means the very thing about her that made her distinct has mellowed out, and Ichika sucks.