I don't read to the comics to think about sex, that's what the rest of the internet is for. Any joke based around a character having sex, even a dog, has little margin for error.
Runners-up: Beetle Bailey, Brewster Rockit: Space Guy!, Curtis, Family Circus, Mother Goose and Grimm, Tank McNamara
Frazz goes back to back. The image of a train driving into Lake Michigan entertained me. It's a nice touch that this would happen if the train went any direction from Chicago that approximated East.
Runners-up: Agnes, Big Nate, Candorville, Dilbert, Pearls Before Swine, Pooch Cafe