Being the initiator, I claim the right to post first 

I enjoyed most the first generation of Need For Speed, the one ending with Hot Pursuit 2 (2002).
Otherwise I've played them all, finished 3 quarters and discarded the last 3 or 4 titles.

NFS Top 5, in my opinion:
1) NFS 4 High Stakes
2) NFS 2 SE
3) NFS Most Wanted
4) NFS Porsche
5) NFS Hot Pursuit 2 (2002)

I remember buying my first PC back in 2000, wich was a mere Celeron OCd at 166MHz, 32MB EDO and a 1MB Cirrus Logic 5446 PCI onboard card. The joy was maxxed out 1 year later when I found the financial support to update the configuration to 2MB (adding 2x512K extensions in the empty slots) and 64MB of EDO. Then I could play NFS 3 in software render and StarCraft with 7 computer controlled players without stalling the system.
Jesus, now I realise how poor my family was at the time.
Anyways, I managed to have someone lend me a Dragon Voodoo 1 and after I've installed everything, I saw the huge difference in graphics while playing NFS 2 SE. The glide worked like a charm (it didn't change anything about the first NFS however - guess it didn't have glide support) and I dunno... everything seemed to catch life. The colours, the fog, the snow... God what I wouldn't give to be at that age again and to feel that difference for the first time.
So that was the breakthrough for me, the moment when I saw why 3dfx was so outstanding (think about it, ironically the company had already been banckrupted when I discovered them).

Afterwards followed the period in wich I started visiting computer cafe's, wich all were running 3dfx accelerators, and discovered NFS Porsche. By all means the most complete NFS ever to be launched and in my opinion, the most graphically inovative.

Then in 2004 I changed my PC config, bought an Athlon 1800+ with a GeForce 4 but replaced the card with a Voodoo3 (also lended to me) in order to play NFS 4 with glide. That's when I found that, besides being very hard, it marked the revolution of the NFS genre: damage, handling, transparent windows, carreer mode... the works.

From the newer generations, Most Wanted catched my eye but not for the graphics and car customisation (of wich I'm not very fond), but by bringing the most arcade enjoyable concept of police car chases. Very exciting game.

ProStreet I found to be the most realistical, the graphics was astonishing at the time, the damage mechanics started to improve and, along with the Shift series, I consider them to be the most simulational. And the hardest !

I can't end the post without giving the top of the worst games in the series:
1) NFS Carbon (dude... drafter... seriously ? If I can't win a race and my drafter wingman can pull ahead of me at any time, why doesn't he go for the win ?)
2) NFS Undercover (weak handling, story, all of it. Weak !)
3) The latest NFS Hot Pursuit (2009 or so) : no feeling at all, weird arcade style.

I for one am sure that there can be nothing done to revive the series and to come out with something that will make me acknowledge I'm playing a racing video game with the perfect balance of simulational driving, delightful graphics, fine music and correct carieer ascension.