When I was in college, my parents moved from MA to FL. We held a garage sale, and I sold my old Sega Gensis, SNES, and N64. I really miss some of my SNES games... Earthbound, FF3 (back when it was 3, and not called VI), Legend of Zelda, Super Mario RPG... I miss those old games! Especially Earthbound, which was hilarious and unique.
I did, however, keep my regular Nintendo. I broke out Mario 3 last year and beat it in like an hour (a lot easier using warp whistles). I also beat the original Legend of Zelda. Ah, good times.
While not an "old" game by age definition, some games I always go back to are Portal and the Resident Evil series. I have replayed those games more than perhaps any others.