I haven't quite decided on this one yet, so I'm going to list some out here and hope the answer becomes clearer by the time I'm done.
Among the top contenders:
Sailor Moon & Tuxedo Mask, Sailor Moon R movie: The love of your life (and your life before your life) has just snuffed out her own soul, but lucky for you, true love and freaky flower nectar conquer all.
Risa & Otani, Lovely Complex: A rooftop kiss that is doki doki wonderful and chock full of the awkward high school romance experience. Nevermind the (literal!) fireworks, they have this super private moment just feet from their friends, who’re all none the wiser.
Hiro & Nowaki, Junjo Romantica (Egoist): After that confession! Guuuuhhhh….. did I leave my insides somewhere? I'd go look for them but I don't want to miss you making out.
….Ahh screw it; I'm tired of trying to decide.
Day 24: Best kiss scene:
Why not? It's the first one I thought of, so there you go.