Burnout Paradise
Very fun, especially online. No licenced vehicles, but still some great vehicles nonetheless with some great sounds (especially the motorbikes). Races are fun, some are easy, some aren't, big online community.
Midnight Club: Los Angeles
Some very nice vehicles, nice customization features, not much customization features, but some nice ones. Races aren't hard, but they aren't as easy as some other racing games. Cockpit view (FTW) with customizable interior (which is nice), Los Angeles has been modelled quite well into it (the places haven't been modelled VERY accurately to where they should be, and Venice Beach is non-existant in it, but nonetheless, still great) and cruising is very nice. Online community isn't that big anymore though, but online is very fun :).
Need for Speed: Undercover
Quite a few nice, popular vehicles, but not very well priced... The customization isn't too bad, designs on cars are quite awkward to do (which is why you'll find not many people have done very graphical designs) due to no Copy function, but it's not bad. Police are VERY stupid in it, so watch out, they are very annoying. The missions aren't bad, the story isn't that good, but it's still fun. Not played it online, so no online comment here, but it is quite fun and the graphics are pretty nice.
My Choice
Burnout Paradise
Much better value for money, much more fun, nicer cars IMO, racing is more fun in it, and the online community is very good for it, I highly recommend it.