I'm not sure about the order, but here are my top five:
Elena and Damon dance and love scenes, "I don't think I've ever seen you more alive." (4X07)
Elena remembers Damon returning her necklace "I love you, Elena." (4X01)
Elena drinking from Damon "Blood sharing is kinda personal." (4X02)
Damon and Elena bed and love scene "I'm happy." (4X08)
Damon talks to Elena about breaking the sire bond "I have to do the right thing, by you." (4X08)