Fallout 3. It expanded what the idea of an RPG could be. It was an open-world FPP game that let you approach situations from just about any position you wanted. Sniper rifles, melee weapons, heavy guns, armor, bolt-action rifles, even flame throwers could be your weapons, or if you preferred, you could use your verbal skills, or stealth, or hacking ability. So much variety, so many things to discover.
On top of that, one of the best settings in all of gaming; dystopian but unique, apocalyptic but teeming with new life-forms. It's a game that creeps into your imagination, where the next step you take is up to you.
I still remember my first encounter with a Death Claw. It was just so bad-ass that I loved it despite being one-hit killed. I remember when I first stumbled on a Super Mutant Overlord in the center of the damn map. I remember getting the Super Mutants and the Brotherhood of Steel to fight so I could loot their stuff. I remember all the fantastic humor hidden under the surface.
I don't know how many hours I spent in that game but, especially when you factor in mods, it was a LOT. So many of the things that I wanted from a game, I got from Fallout 3, even though I didn't know it until I played it. It was truly a game-changer for me and I want more like it.
Damn if I don't want to play it again.