NFS 3, 4 and 5 are imo the best NFS'es of all time.
NFS3 for the small but nice set of cars, the racing against the police, the fun and nice tracks, the great music (hears Romulus 3 while seeing brand names flying by....) and the narrator with the car slideshows and the track explanations.
NFS4 for the level it went up compared to NFS3, the nice effects such as interior illumination, the career mode with High Stakes challenges and vehicle tuning, the inclusion of NFS3's tracks, the even better police mode where you're actually a team, it was fantastic. And once again, the great soundtrack.
NFS5 for its atmosphere, the fantastic soundtrack, the very well done Evolution mode, the great and challenging Factory Driver mode, the huge amount of Porsches and available tuningparts, the tracks, the skies, the environments.... Everything was great.
As for the rest, I played NFS2 as well but it never caught my imagination as 3 4 and 5 did, and from NFS6 on everything went downhill. The cars in 6 felt unnatural, graphic details weren't good (such as metal interiors on some cars), the extreme steps back looking at what you could do in/with cars in NFS5 and in/with cars in NFS6... and after that, it was from one plastic party that makes Barby jealous to the other horrible storyline. NFS Underground 2, well I finished it and it was okay to play, but that's just about it.