I have found going back to the old games you used to love isnt always a great idea as sometimes they just dont feel as good.
Some of the older games i miss playing though are:
Diablo 2 (multiplayer before it was ruined by hacks/dupes)
Resident evil 2 on the PSone (brilliant game and completed it loads to unlock most of the stuff).
FFVII The best game ever made.
SFII the best fighting game ever made.
Killer Instinct 2nd best fighting game ever made. 130 hit combos ***
Used to come home from school at lunch with my mates and try figuring out new moves.
Super Mario World - Loved this game and i played it the other day and it is still as good.
Marioworld on the original gameboy, i actually played this on my nieces gameboy the other day adn was supprised that i remebered every little hidden thing despite having not played it for about 10 years.
Starwars galaxies (pre SOE screwing it up) Before all the upgrades and crap it was the best MMORPG ever, apprently there is going to be an emulation server made that will run like the original version but i just think it will be the same now without my guild or massive stash of crafting materials
Played Sonic the other day and that is still fun too. Altered beast and golden axe sucked though