I've been watching a lot of retro and budget build videos on YouTube recently, which has got me thinking back to my previous GPU history.
Top of the list for me is the no-name OEM 3Dfx Voodoo I picked up from a guy who traded out of a lockup. I remember no-one had stock of them at the time as they were newly released. I was desperate to get my hands on one so took a punt and hoped it worked. My jaw hit the floor the first time I saw Quake running under OpenGL.
Second to that is my Gainward GTX670 Phantom. It's still doing solid work in my son's PC and is happily churning out frames at 1080p Medium settings in most titles.
I paid £15 for it when it was only a year old from a serial upgrader who just wanted shot of it after buying a 780Ti. I'd say it's given the most VFM of any hardware purchase I've made.