Voodoo 2 8MB, Voodoo 3 3000 AGP, Voodoo 5 5500, Gainward GF3 64MB, TI4400 128MB, and X800XT.
Liked each of these because each gave me a huge performance increase across the board compared to the card they replaced. My absolute favorite was probably the upgrade to the Voodoo 3 3000. Got the performance of SLIed Voodoo II and no longer had to own separate graphics cards for 2D and 3D. 2D output was amazing too. Must give respect to the Voodoo 2 though, as it is what started me down the 3D gaming pathway I am still on to this day. Paid $212 for that bad boy back in the day when they were still going for $249.99 MSRP. What a deal!
Someone above made a comment about being able to "see" games when they got some 3D acceleration going on. Quake 2 was kind of a moment for me, but getting the 3dfx patch to work on Need for Speed 2 on my Voodoo 2 was a "OMFG WTF!!!! Ooohhhh preeettty graphics...droolz" moment. Must thank the Voodoo 2 for displaying Final Fantasy VII on the PC, which got me hooked into RPGs (although sadly, I pretty much have to play RPGs on my PS2 anymore).