TrackMania² Canyon or Valley or Stadium

I only have Stadium myself, plays much like the free game I'm used to. Not sure how the others are different, other than the map types.

Stadium is very similar to Trackmania Nations, which is free. It looks prettier thanks to the new engine though. That's all, it's only worth getting if you really want it.

Canyon is pretty fun, but the car isn't as tight and precise as the Stadium car, which means that you have to watch the weight transfer and easy drifting more. The driving is more 'loose' personally, but that doesn't make it any easier, because the cars still move really really fast and the online tracks people come up with are punishing. I recommend it if you just want to get one Trackmania 2 title.

Haven't played Valley yet, but if it's similar to the Rally environment from United then it'll be surprisingly technical and frustrating. Which is a good thing. You could try this too, but I have no personal experience.