As someone who didn't feel particularly attached to any quint in particular, I can say objectively that the way the manga was written anyone could have won. Every fanbase had arguments to support their quint, and Negi played them for all they were worth. The quintuplet waifu wars were the main selling point of this manga after all. Family relationships? Who cares. Dreams? Boring. Searching for an identity or independence? I sleep. From day one it was always about who the self insert's bride was, and the way Negi plotted this manga made the five of them viable choices according to how one interprets the manga, here being some examples, though I can't claim to speak for everyone here:
Ichika fans saw a story of a girl crossing the line for love and atoning for it, and expected her to be rewarded for doing the right thing and getting over her unhealthy way of expressing her attraction.
Nino fans saw a girl warming up to the guy, getting over her first crush and growing to love him after understanding him, kinda like Golden Time. They expected her efforts and aggressive stance to bear fruit.
Miku fans saw a story of a girl growing up and learning to stand for herself and do the things she likes. She radically changed for the better, and they expected her growth to resound in the MC.
Yotsuba fans saw a love story that linked the girl and the MC from a very early age. Her development was very intimately tied to that early crush, with a theme of "destiny" not as blatant as Nisekoi but still very in your face. Her fans expected it to blossom in a full love story.
Itsuki fans... eh, got nothing here. I guess you could argue that those who expected her to end up with the MC are already jaded fans who expect first girl to be the default winner, not that I could blame them, myself having been one of them. Negi took care in not showing his bad side, always her being the calm and responsible one in contrast to her sisters' desperate attempts to grab the MC's attention. Tired savvy readers saw her winning by process of elimination, just like what happened in Nisekoi.
Arguments that are valid, since they have been used in other romcoms ad nauseum. The problem here, like every other romcom that turns into a trainwreck near the ending, is the MC. He remained passive the whole duration of the manga. Romantically speaking, the outcome would have felt like a copout regardless of who won because we never saw him favor one girl over the others not even once. Not subtle hints, no likes or dislikes or things that would have made him realistically bond with one quint in particular. The biggest bias he might have had that was the whole Rena business was smoothed over as soon as it became too dangerous to upset the status quo. His affection points were raised by the five of them at roughly the same time or by none of them at all. Any semblance of a hint was nebulous or very open to interpretation, which only served to stir the fire of already very passionate fanbases.
In the end, you supported the quint who better fits your interpretation of the story. I expected a second Nisekoi, a manipulative collection of old cliches in a fancy new wrapping that would end in a shitstorm and that's what I got, so I'm pretty satisfied myself.