Mario Kart DS
Basically the culmination of every good thing to happen in the Mario Kart series since its inception. There are 32 tracks, though a few can't be played in WiFi mode due to latency issues: 16 Nitro (which you may remember was the original codename for the DS) tracks, which are brand spanking new and all very well designed, and 16 Retro tracks, collected from the past games - four from Super Mario Kart (SNES), four from Mario Kart 64 (N64), four from MK Super Circuit (GBA), and four from MK Double Dash (GCN). Reliving the SNES tracks is ultra-sweet, and they've resurrected most of the best tracks from the old games.
There is a fair amount to unlock through winning Grand Prix cups, and though I personally did it all in two days a saner person would probably get significantly more time out of them. The game starts out with eight selectable racers: Mario, Luigi, Peach, Toad, Yoshi, Bowser, Wario, and Donkey Kong. Four more can be unlocked: Daisy, Waluigi, Dry Bones, and ROB (the plastic NES robot!). Each character starts out with two selectable karts, and in addition to being able to unlock a third for each racer, you can unlock the ability to select any kart for any character.
MKDS also has a mission mode with some pretty fun challenges - collect all the coins, snipe all the fish with green shells, drive backwards through this map, push this boss off the platform, et cetera - but as far as anyone knows completing the missions has yet to unlock anything exciting. At any rate it is a fun distraction and a way to hone certain driving skills in a non-racing manner.
And online play is pretty easy and fun. Unfortunately you will inevitably encounter someone online who completely whores the power slide, e.g. zig-zagging in straightaways to get perpetual speed boosts. Personally this bothers me, as I refuse to believe that any human being can actually pull this strategy off, but in any event that's why the Friends system was invented for Nintendo WiFi and it works excellently.
Progress: Complete