I agree Smithers is too obvious. There have been hints that Patty is less interested in men than Selma. Selma actually got married at least twice, while Patty gave up the only guy interested in her (Principal Skinner) to stay with Selma. But again, too obvious.
Milhouse is a little weiner, but he has a crush on Lisa, as does Ralph. Professor Frink, Apu, Cletus, and Dr. Hibbert are all married. Jimbo, Nelson, Kearney and Dolph are always together... how gay is that? Lenny and/or Carl, or maybe someone more obscure, like Groundskeeper Wilie, Dr. Nick Riviera, Comic Book Guy or Bumblebee Man, are all decent choices, but I am going to go out on a crazy limb and say Miss Hoover, Lisa's teacher.