Given that I was the one that requested this thread, I probably should be one of the first to vote.
I like the entire main character cast, and I think that they all work really well together.
For me, though, the strongest spices in the final dish are Ritsu and Yui. Those are the two characters that I simply find the most entertaining.
Ritsu's edge, imo, is that she has a bit more of a "down to Earth" side to her than Yui does. You probably get more "serious Ritsu" moments than "serious Yui" moments.
Yui's edge, imo, is that I actually find her to be an excellent stand-up comedian whenever the anime uses her in an "addressing the crowd" role. I've even compared her to Will Ferrell in this regard.
For me, it's a really tough choice between these two, but I have to settle on Yui. Yui is the wild-card, the goofball, the one who's totally unpredictable. At least that's how I see her. So that's why she has my vote in this.