It's an excellent cast, and I liked all of the major characters, but for votes like this to have any meaning at all, I think it's good to limit yourself to four or fewer choices.
So my four were: Kyousuke, Haruka, Komari, and Kurugaya
Kyousuke is a fantastic and well-rounded character. Very entertaining to watch, and with a good blend of coolness and hot-bloodedness. As amusingly silly as he could be at times, it helped that they kept him
just serious enough for the more emotional scenes to work well.
Haruka, Komari, and Kurugaya are the three major female characters that had the best combination of personality appeal and sex appeal to me.
Haruka strikes me as the perfect representation of the Key heroine. Very colorful, slightly eccentric, playful, amusing, cute, a bit childish but not as immature as the harsher Key critics would have you believe. Her arc was also one of the more memorable ones to be sure.
I know some didn't like Komari, particularly her voice, but I honestly found her quite adorable. Granted, her arc was perhaps the most underwhelming of them all, but I think Komari made up for that by being probably the best "support character" of the heroines in this show. Her level of dedication towards Rin was genuinely touching, and this yuri fan certainly enjoyed the Rin/Komari scenes in this anime.
Kuruguaya is simply the ultimate dream girl. Her arc, though short, was very strong and memorable. I also felt she had the best romantic chemistry with Riki of all the LB heroines, including Rin.