Perhaps not the best place to ask this but since we don't have a general NFS section... I think the original NFS Most Wanted is the best, but I also liked NFS Shift games, which were the only simulators in the series worth playing. I also liked Carbon, but that mostly looked and felt like a copy paste job.
NFS : Hot Pursuit is what I remember playing. And it was one of the good game. And all the cars were good too. Some of the levels were fun. And the best part was the graphics. The graphics is lot improved now.
Did you play the original or the 2010 remake ? I assume the remake the since you mentioned good graphics. I never did play the original since it came out before I started gaming, but I did really like the remake.
Mine has to be Need For Speed: Underground. Maybe it's because of the time I spent playing it at my grandma's house so many years ago. It was lagging so much but I couldn't stop playing it. I had so much fun with that game and the other games didn't give me that same feeling like Underground did.
Same here, its basically the first NFS i played and had lot of hours spend with the game. Its also a learning game for me specially in control and features.
NFS Most Wanted (the original, not the remake) is pretty much a more polished and better version of NFS Underground, so play that if you haven't already. Actually I think it's the most recognizable game in the series hands down.
My favorite would be Need For Speed: Most Wanted. It was the second Need For Speed game that I've played. I liked the plot better than the previous games and I also like the street style of the user interface.