Nfs ProStreet was the worst of the series. It doesn't even work right, but even if it did, it's just plain crap.
Carbon was also boring and was lacking fun. Also, I never really liked Underground 1 or 2, although they were fun to play they've become very boring, very fast.
From all recent NFS, Most Wanted was the best really. Going back in time, I liked NFS Porsche 2000 a lot and quite all the others that were until this. Hot Pursuit 1 was superb, NFS at its best.
For the moment, GRID is absolutely beautiful, a nice substitute for NFS and Test Drive Unlimited it's also a very good racing game. Let's hope that EA will manage to create this year a better NFS, although I really doubt it.