I have all of NFS since the Road & Track one and I will keep getting the newer versions as long as they make them.

I basically likes all of them as I saw how they progressed in graphics, driving techniques, and challenges they put you against throughout the series. Carbon is a growing love if you ask me as it has a nice mix of cars, new features to explore and a lot of fun competing with my brother on who is the best driver between us
My favorites are
NFS2 => Mclaren F1

+ Lotus Esprit V8 + all other cars basically: great fun
Road challenge = High stakes => lots of cars and challenges + good graphics + NFS3 tracks
Porsche 2000 = porsche Unleashed => My brother is a porsche fanatic and this is such an excellent release, graphics, tracks, cars, variaty of challenges and the only release where I couldn't actually beat my brother at all
NFSUG 2 => new open world racing, SUV's, lots of cars,new challenges, parts shops and all the bling bling stuff
NFSMW => graphics, cars, driving in different environment, large map, cops always a good thing " only need to be able to play as a cop ", although the damege is non existant, the cops damage scenes look nice and wish it was implemented in the playable cars themselves even if it was just graphical damage without actual effect on the car performance.
Ahhh I'm obsessed

well its OK by me