Fallout 3 and Fallout: New Vegas are two of my favorite games of all time.
Why? Probably because it's not a game that holds your hand and leads you through until the end. Options are everywhere, options that really matter. Want to talk your way through most of the game? You can. Want to go stealth and specialize in silenced headshots? Want to go hacking or open safes? Want to wear armor and just mow everything down with a mini-gun? I love that freedom.
I also loved the exploration factor, where you never know what is just over the next ridge. Might be a super mutant camp inside an abandoned Vault, or a settlement of survivors, or even a pack of Deathclaws. You might die horribly and realize you are way, way out of your league, but that really want to explore this area when you're more powerful.
I also loved the companion factor, the costumes you can wear, and all the awesome mods.
