hmm, not sure i can pick one thing out and if i did it wouldn't accord with my current use of it. I had a TNT2 card for a long time and took me through many games back in 90's and very early 2000's - so maybe that for nostalgia. such an ordinary card but its like an ordinary car to someone who wants to visit places, it enables that to happen and so fond memories are formed. otherwise its the whole PC i bought with P166mmx, i don't have anything older now having moved and lost/given/thrown things over the years - so it is the oldest hardware i have left

I thought about this for a long while. I don't have a -the- most favorite pieces of retro hardware. There are multiple that I like a lot but there isn't a single one lonely at the top for me.
Stuff that I like are good boards with 440BX chipset or i815 chipset, as these tend to just work.
Cards like S3 Virge, Verite 2100, Voodoo 3, GF4MX (and GF2MX to a slightly lesser extent), GF6800 and GF7600GS with large passive heatsinks which I juryrigged active cooling onto (both cards roughly same performance when used in WinXP).
Simple PS/2 Logitech mouses with (for these days) terrible dpi and simple Dell or Compaq branded keyboards as these were decently robust and very cheap.
Arctic Cooling Copper Silent 3 for sA and copper based stock sA HSFs when used for s370.
Coolermaster Centurion 5, Chieftec Dragon and AOpen H600A cases.
Pentium MMX 233MHz, Cyrix MII, AMD K6-3 and Coppermine 1000 (and Tualatin) and A64 Venice cored CPUs
This doesn't mean that anything not listed here means I don't like it!

I'm really kinda impartial to sound cards. I mostly care about whether or not I hear something from the speakers at all and am not an audiophile (I guess Im used to so poor quality sounds back in the day that my standards for sound quality aren't very high), as long as it makes sounds without too much annoying bugs, I'm usually content with it.