MKDS. I guarantee you it's the best Mario Kart game to date.

User Rating: 9.3 | Mario Kart DS (DS Bundle) DS
You have not played anything like this before! The previous Mario Kart games really surprised. There was nothing more satisfactory than flying forwards with a Chain Chomp or blasting the leader with a blue shell in Mario Kart: Double Dash. But this really takes over. Firstly, there are new tracks, in addition to the best tracks from the SNES, N64 and GameCube Mario Kart Games. It is much more challenging than the other games and also a lot more rewarding, with many karts, tracks and characters to unlock. The classic multiplayer balloon battles and Shine Runners along with standard races make it a must have for all Nintendo DS owners, but if you do not own the game, you can still play with someone who does via the DS Download Play.

You can also play against other people with the game, allowing each person to choose their racer/kart and giving a wider selection of tracks. But the main multiplayer experience if via Nintendo Wi-Fi Connection. You can play with anyone around the world and play on an agreed track.

I gave this game a Hard difficulty rating as some parts of the game can be very challenging, such as the higher engine classes of Grand Prix, but most importantly, completing all of the missions, especially with at least 1 star rating on each, which gives you stars next to your name on multiplayer corresponding to how well you did. I set the learning curve to high because anyone can pick up the game and have a blast, but to beat the really competitive veterans via Wi-Fi you need some skills, such as snaking and boosting around corners well.

I only have one thing else to say. If you have a DS, and you don't already own this game, get it, and you won't regret it.