Definitely Metal Gear Solid 2, Quake 2, Summoner 2 and (mostly) Unreal Tournament - the game that finally convinced me to get UnrealEd and abuse it to the point of overheating my first laptop many, many times.
Secret of Mana 2, Diablo 2, Worms: Armageddon and ES: Morrowind are the games I messed with the most with editors / emulators. Some various GB games I had on emulator on PC, I would go in to the "code" and change things around, felt cool bro.
Deus Ex was the first game that really showed me what games could do. There hasn't been another game that's surpassed it yet, but it was Half-Life 2 was the game that really got me into game art, though (after some failed attempts at Quake 2 skins).