I think it was "I like girls with big fat tittys, I like girls with deep viginas!"
(taken from the artical)
Kenny's lines in the song, like the rest of his speech in the show, are muffled by his parka hood, which covers his entire face except for his eyes; however, Kenny's lines have been revealed by South Park Studios. In seasons 1-2, he says "I love girls with big fat titties, I love girls with deep vaginas." [9] From seasons 3-5 he says "I have got a 10-inch (250 mm) penis, use your mouth if you want to clean it."
From season 7 to the first half of season 10, Kenny says "Some day I'll be old enough to stick my dick in Britney's butt." [11] Starting with the second half of Season 10, Kenny's lines changed again; however, the current lines are not known.