It's really hard when they're all fabulous in their own way. Senpai seems like my favorite (brains is a huge plus), but all three of them have a super important trait: Kindness.
Main Girl has something that maybe goes beyond that, that she's selfless, but maybe to a fault because she isn't honest with herself.
Honestly he could end up with either of the 3 and I couldn't get mad, there's no clear winner, they're all wonderful, kind, smart, hardworking, and caring women.