I don't know if it is the best kiss, but Pretty Woman has a good one.
An extremely rich man hires a prostitute to be his 'escort' for the weekend as he closes a tremendous deal. Eventually they fall in love, even though the agreement was 'strictly business'. At the end, he rides up in his limo, through the moonroof, and climbs the 'tower' (or a fire escape) even though he is afraid of heights, and kisses his prostitute.
I thought it was a good one!