I have a top three of on-screen kisses -- each of these totally drew me into the emotions of the story, and even now, decades later, I can close my eyes and see them unfold.
#3 -- Andie and Blane at the end of Pretty in Pink -- he practically worships her mouth in that kiss (pic from a different kiss in the movie, but it works).
#2 -- Watts and Keith (the human tater tot, hehe) in the middle of Some Kind of Wonderful -- I mean, really...would anyone who's seen that movie disagree? That was a seriously hot kiss.
And my favorite of all time, #1 -- Jessica and Jim about 2/3 through The Man from Snowy River -- Wow...seriously, that kiss as the sun sets behind them, while they sit on his horse...*melts* I so wanted to be kissed like that! (pic from moments before kiss)
How about you? Which kisses have you seen which affected you?
ETA: pics :)