I always liked Yui, she is a kind person and always given Hikki a chance, her crush on him has been around before anyone else's and i think they are a good match, they had things under the surface which make them similar, mostly how explored in S1, their level of "belonging" is low, Yui just is a sheep and follows crowds to be wanted whilst Hikki says he thoughts and is hated, full circle really. Also, Yui has been running this race much longer than anyone else, she deserves some love in the ending. So why don't i want her to win? Well when S3 was announced i decided to re-watch s1 and s2 (start of this year) and i noticed, whilst Yui is kind, i still think many of her moves are to please the many over the few, not in a good way, Hikki pleases who asks for help, Yui on numerous times i felt was a bit harsh or out right mean to him, despite it being un-warrented, this i found my learning to Iroha.
Iroha - she is the late comer and wont win, but she kind of feels more fitting to win. Despite her who knows how many rejections of Hikki (which was one sided rejections considering he never did anything more than be kind, sympathetic or just be outright human), even she came to realise the hard nasty snake the world sees Hikki as is actually a kind fluffy dog under it all who just wants some love and attention, she accepts his "harsh" side whilst seeing his true side, a bit like Yui. Where she differs from Yui is, she isn't bothered about please many, she does what she wants, she bats of any romantic interest into a joke to hide it but weirdly that only works more. The two seem to work together, their interactions feel natural and i feel the help Hikki gave her, has brought her to where she is and she appreciates this, a little more than the average competitor. She is the balance between Yukine and Yui and that i think is why she deserves to win, also, the other two are aware of one anothers feelings now, whoever wins will only make the loser sad, sure it is probably beyond the series scope but i hate the thought of a sad dejected Yui or even Yuki.
Yuki - I never liked her from the start, her rough shell wasn't funny like Hikki or even associable, she was really just bitchy for the sake of it. well, this is how i first thought, after re-watching i came to see her as a kinder person as i came to see the good which came from her. Her and Hikki is likely the real ending but she does feel made for him... the only issue is, the two are like twins, not in looks but they think identical, the humour is lot and they feel kind of dull, heck they hardly spend time with one another with someone else around anyways, and it never feels normals. I think this is because as they say "opposites attract" and these two aren't that, they are identical, with no differing in them outside minor things, i see the most bland relationship. Granted my opinion of Yuki changed a lot in my re-watch, i still don't want her near the finish line, i think Iroha is best fit for it though the least likely, then Yui, then Yuki.