Who out of the main 4 (Zim, Gaz, Dib, and Tak) is the best kisser?
Doubt any of them do. The only person whom I can imagine giving someone a kiss or smooch is Gaz to their dad as familial affection. But even that feels impossible. The best affection both siblings give to their father and vice versa, is communication(even tho it lacks greatly as shown throughout the show.) There may have been a time they all exhibited physical affection but that's hard to remember. To me, I mean.
My best answer would be Tak. At least she'd probably have the most idea of it than Zim. Not on physical though. I like to imagine Tak is very knowledgeable and actually has done her research when it comes to human culture.