Looking back at my purchase history, I used to upgrade almost every 1-2 year. It wasn't until recently that I started to slow down on the upgrades. I have noticed I tend to buy mainstream cards from NVIDIA and top end cards from ATI/AMD. NVIDIA cards purchases are more frequent where as ATI cards tend to last longer.
Voodoo 3 3000
GeForce 256 DDR
Voodoo 5 5500
Radeon 9700 Pro
GeForce 6600GT
GeForce 7800GT SLI
Radeon X1900XT
GeForce 8800GT
GeForce GTS250
GeForce GTX460
Radeon HD6970
Radeon HD7950 (died after a week, returned).
GeForce GTX970
I think overall, I've been happier with NVIDIA cards in general. But buying mainstream cards always left me wanting more from higher tier cards, which lead to more frequent upgrades. My current 970 runs everything smoothly and doesn't make me feel envious of Titan, Fury X or 980Ti cards. I'd only upgrade it when I see 100% better performance from a card of similar range or cheaper, and with Freesync support.
Radeon cards have been disappointing to me. I don't really have hard numbers to quantify this, but they never felt as 'smooth' as NVIDIA to me despite topping benchmark charts. It's just the lasting impression I got out of them, but that's probably because I held so long on those cards.
The most satisfying cards I have owned are:
Voodoo 5 5500
It may not be the fastest cards of its generation, but it ran glide games the smoothest, and I never had hiccups with it. It is just one bad ass card of its time.
GeForce GTX970
I think it is truly the first card I have owned where I could crank everything up and still get butter smooth frame rate.
The most disappointing purchases:
GeForce 256 DDR
That card was a disaster when it came to 3dfx glide games The inferior Voodoo 3 would render perfectly smooth glide games when this one was a choppy buggy mess.
Radeon 9700 Pro
I know it was a revolutionary card for its time, but I didnt think it lived up my expectations. It was a fast card no doubt, but it didn't feel as smooth coming from the 3dfx glide experience.
GeForce 780GT SLI
Baby, that set up was smoooooooth as butter from what I can remember. Too bad it had to be SLI.
Radeon X1900XT.
It's the most I ever spent on a card. I think I bought that card for around $600 at the time, and never got much gaming value out of it. Something about it never lived up my expectations, and I don't ever remember running it smooth in any games.