Alright, let's see:
Voodoo 1
Blew my mind, simple as that. Quake was really impressive, as well as some other games like Fifa 98 which looked rather bare in Software mode.
Voodoo 3
The best all-rounder of the 3Dfx cards. Plenty performance with great compatibility. The go-to card for most late 90's retro builds.
Radeon 9700 Pro
Marks a pivotal point in GPU history, but I'd never look for one nowadays.
GeForce 6600GT
Legendary card because of its value for money. It usually was well ahead of the FX 5950 for a very sweet price. I'd also include the lesser 6800 models like the LE,XT and vanilla for their unlockability, meaning you could potentially make your card run about as fast as a 6800GT.
GeForce 8800GTX
Milestone in performance gains, IQ and AA improvements. A lot of praise should go to the 8800GT as well, which brought about 85-90% of that experience to the masses.