My favorite is Mario Kart 64. This may be partially due to nostalgia (and I probably played it more than any multiplayer game (excluding perhaps mario Party 2 and Super Smash Bros. Melee), but I also feel it was the most innovative when compared to it's predecessor. It obviously kept the same style and concept, but it great improved upon the Super Nintendo one. Double Dash was basically a rehash, IMO, and Mario Kart Wii was basically the same thing, just with updated graphics and a new style controller. I ahven't played the handhelds, so I can't say anything about them, but I feel Mario Kart 64 simply has something special about it and remains the greatest (and most addicting) Mario Kart game to date.