The only 'retro' games I play with any sort of regularity are Goldeneye and Mario Kart. Goldeneye especially- 4-player sessions are as frequent now as they were six or so years (edit- ouch!) ago.
Repeatedly playing single-player games through just bores me, really. I like to be surprised. Fire Red, as said before, is a good example. It was still good to play through again, but nowhere near as fun as the first. After beating the elite four for the second time, I just left it- whereas on the orignal I played it tirelessly until I had six level 100 'mons and all 150 in the pokedex. I guess I just had more time on my hands in those days.