I don't say it often, but this is the best!
Unlike Oblivion, you never get bored with the main storyline. "Okay, I've closed another Oblivion Gate, how many more GEEZ!" (not that I hate Oblivion either,) Fallout keeps you interested, it keeps you going into different buildings you might have at one time recognized to meet people you'll want to meet again. The enemies are fun to fight, and the ambiance doesn't grate on your nerves as I've noticed the Elder Scrolls counter parts usually end up doing.
Fallout 3's only real fault is it's rather confusing map. It seems like you should be able to make it from one end to the other, but usually you end up needing to take a subway tunnel or something, which you'll eventually learn, but at first it may be a little frustrating. Not to mention that although the outside world is beautiful (it's it's own post nuclear way,) the indoor graphics sometimes leave things to be wanted for. However you'll hardly notice as you mow down mutants and zombie creatures.
If I could sit in a chair the rest of my life and play one series, I'm pretty sure Fallout would be it. Despite the few faults, this game has challenged me, made me feel great, shocked me, and then made me feel accomplished.