I nominate Cupcake to be gay lol. Of all the random people on the Enterprise, he's has the most "screentime".
I wouldn't mind Sulu being gay, but he did have a daughter in Generations.
In terms of attractiveness, I'd choose either Kirk or McCoy. I don't see much point in making a minor character gay. If Star Trek has a gay character, it should be someone important. I'd like Pike to be gay in the sequel. He's a great man and would be a great role model for gays, not that they need one.
Besides, if a Star Trek character was made gay, Paramount would make more money than lose it. That is to say, they'd gain more fans than they'd lose.