Burnout Paradise is probably the easiest and most fun game of the bunch. I got the platinum trophy and didn't even work for it, it's that much fun, and so much to do, lotsa DLC (recommend you download Big Surf Island)
Midnight Club LA is like a poor man's version of Burnout Paradise, with no awesome crashing, but you can customize your cars a whole lot, which is probably the only arcade racer to do that. Definitely not hard though, there are difficulty settings, too. They did a pretty good job representing LA, and if you liked the previous ones, can't really go wrong here.
There are games halfway between arcade and simulation. GRID is probably the best one. You can start with racing contracts right away, basically driving for someone's team in the game, meaning you get to drive awesome cars from the get-go. I thought that was pretty cool that the first race in the game is in a fully-tuned Viper (the one on the cover).
If you like offroad racing, like others said, Dirt 2 is simply amazing, although they overdid it on the "You're a cool dude racing with other cool dudes" aspect, it's lame after a few hours. Very polished game though. Another good one is MotorStorm, which is probably the hardest game for me, but it is pretty awesome to drive those big trucks and boost your way though jungles and mountains.
If you like futuristic racers, Wipeout HD is probably one of the best PSN games out there, let alone a good racer. It doesn't have nice cars, but the thrill of racing at several hundred miles an hour is pretty hard to get from other games!
I would avoid the Need for Speed games (especially Undercover, Pro Street), this series has become lamer with time, I rented both and hated them: yuck.
