The Buffy/Riley kiss in Hush is painfully underrated. Big, romantic and a great tension capper with the silence.
The Willow/Oz kiss at the end of Phases is perfect. A sweet, affirmation of romance that I personally needed after the pain of Innocence. Plus not only did the kiss advance the Willoz ship, it was key in Willow's character development to go from "That won't make me a slut?" to taking the kiss for herself.
Anya just pushing herself with delirious happiness on Xander's lips in The Gift after he proposes is one of the most ebullient, full of vivaciousness kisses that I've ever seen. Very Anya.
For friendship kisses, Buffy kissing Willow's head while Willow weeps in Buffy's lap after Oz cheats on her is so sweet. Accompanied Buffy glaring at unconscious Werewolf!Oz.