I haven't played the first game. I have played Point of Impact on Xbox and GC and it was a pretty cool game, the crash mode was fun. The normal races were incredible fast, but the track design was seriously lacking. There were rarely tight turns and a lot of times you were driving straight for like a mile. However the boost system made it interesting to get high burnout combos even for a complete race.
Takedown was vastly different in that regards, using the boost all the time wasn't possible anymore (because dodging, drifting and the like give you way too less boost, to use it all the time) , so one of the coolest aspects of the series was lost. Due to the rubberhand AI in Burnout games you can't get away from your rivals more than 2 seconds, so there wasn't much to do during the races. You drove along without using the boost, when a CPU showed up, you instantly used the boost to smack the other car into the rails or in the traffic which filled up your boost gauge again. Repeat until race is over, this became boring quickly.
Legends on the PSP was a compilation of tracks from Burnout 1-3, but with the gameplay of 3, so that game wasn't that hot either.
I haven't played Revenge, because simply watching it made it clear that the gameplay was very similar to Takedown, just a little more simplified. Now you could crash into cars going in the same direction as you without actually crashing your own car.
I haven't played Dominator.
Bottom line, of the 3 games I have played (Point of Impact, Takedown, Legends), Burnout 2 is clearly the best one.