I don't think Lickitung would be the best tongue kisser even without taking into account the pain and numbing because his tongue is simply too wide. It would definitely have to do a lot of uncomfotable folding to even fit, and that's not even taking into account the ugly side-effects of its saliva.
I think the best kind of tongue for tongue kissing is a long, prehensile and slightly more more tubular-like in shape with a tip that tapers in the end. Tapered is the best for kissing because it makes the tip very useful for teasing among other things, and it also eases you in for the thicker parts of the tongue. Max girth would be up to personal preference, but i think something just slightly thicker that a Human's tongue would be best. Unfortunately i don't think any current Pokemon fits that description. Greninja has probably the best tongue so far at least. It's girthy, but not SO girthy. It's very extendable and i'm pretty sure its been shown to stretch many times longer than Lickitung's 6ft taster, but its also retractable so it's not much of a hassle to deal with. It's completely prehensile and strong enough to lift multiple trainers at the same time too. All of this without the side effect some other pokemon have. My only issue with it is the bulb at the tip of the tongue, but even then that could be a plus for some people.