I've spent a huge amount of time on TMDS already (unlocked all the tracks, bought almost everything) and I still have tons to do. Even when I complete it 100% I have the level editor to mess with, which is amazing. Mario Kart DS was great when it came out, but it got old fast. Snaking was fun, but there just wasn't enough to do after 100% completion. WiFi was really boring IMO, and outside of the Grand Prix, there wasn't much to do.
I think that you guys are forgetting DKR, which would have been SO much better if it wasn't for the stupid touch things before each race.