I always thought Yui is best girl and the most suited to Hachiman.
But on the other hand, she'd be able to recover and find her own happiness in the event she's not the one. I trust her on this.
Yukino, on the other hand, is a lot more brittle.
Yui would give ordinary happiness. Yukino would give interesting times.
So, in the end, it's up to Hachiman's choice. Or he chooses neither and just helps them because he thinks he's unworthy.