I mean old like Sega Rally Revo. This is not my video, but it sums up my experience playing in deck. The graphics doesn’t feel too old when compared to today’s games and I’m not sure why but I always get above 100fps even i limit deck settings to 60.
Oldest one I boot up regularly is Super Mario World I would say. On a less frequent occasion I do also play Tecmo’s Deception (PS1). I do like going back to Gradius for the SNES as well.
Contra and Contra 2 are fun, but very challenging games I will occasionally bust out.
PC gaming wise I have been playing City of Heroes and Diablo 1.