It's a toss up between Sly/Bentley and Drake/Sully for me. Both of them are such entertaining and endearing characters, but Bentley and Sully both put so much additional likeability to them both as great supporting characters.
An honorable mention would be Calypso from Twisted Metal 2, who I really wish had more than just ending cutscenes to flesh out his charismatic and sinister characterization. Grasshopper's ending made him into a very likeable villian in TM2 and Head-On.
PSN: MistahRattlehead Official Eddie Riggs of the PSASBR board