Midnight Club LA is the best street racing game out there.
First there's the city. It has a day and night cycle, weather, and unlike NFS is lively. I remember playing Underground (also a good game) and the traffic looked bad and drove like robots. In this game, the traffic makes errors, and gets angry if you stay in there way for too long or crash into them. There are also pedestrians. LA has a realistic feel, due to some famous landmarks, real shops (Pizza Hut, 7/11, Gamecrazy, etc) and the inclusion of licensed cars (by GM). It's split into 5 "sections": Hollywood, South Central, Downtown, Hills, and Santa Monica.
The cars in this game are fully customizable including bumpers, tires, license plates, interior, neon, hydraulics, vinyls, and more. Some cars even have a convertible top that you can take down while driving. You can easily feel the difference between a Lancer and a Challenger, so you can pick cars to fit your driving style. Another thing missing from many racing games are bikes, which this game includes, although only 3 (4 with DLC). You can also drive using an in car camera, to enjoy the masterpiece interior that you have created.
The online is good, and includes things like multiplayer races and free roam, which are expected. Two features that I really like was a Capture the Flag game, and a "Rate My Ride" system. This is good for those of you who like spending time customizing your ride. You can upload one car at a time to the system, and have other people rate your car, and buy it if they like it (you get 10k for each sale).
The police system in this game isn't as in-depth as the one in Most Wanted, but it still provides some fun. If a cop catches you with a minor offense, such as running a red light or speeding a little) you have the option to pull over and pay a fine (or take off again while the cop is walking towards your car). Chases can be a bit difficult if you have a slow car, but as you get farther into the game, you can lose cops fairly easily.
If you are a fan of street racing game, this is a MUST buy. I've had over 150 hours of fun with this game, and still go back to play it every once in a while.