well... Fallout 3 is my drug right now... i just cant seem to turn of the console, and considering i have never played the other two fallout´s i was inmersed in the story rather quikly after a few days of trouble (i wasn´t sure what i wanted to be so i scratched my character like 5 times before i came to the conclussion i wanted to be the super brain guy with 100 med 100 sci and fairly 75-80 speech barter)
i am lvl 22 now... and i damn well hope i can never finish that game, i love every single inch of that wasteland, i remember the time i left vault 101 for the first time i was all "OOOOH, AAAAH" and since i have never played oblivion or any other RPG as vast as this one i felt completely awestrucked by the sheer amount of detail in the game, of course some things repeat themselves quite a lot, but still it feels great every time i look into another damn "point" in the map (i recently aquired the "explorer" so i could see all placess in the map)
discovering and investigating is amazing, and the first time i wore a power armor felt awesome.
also, the weapons system and the repair system and well... almost any game play they added into was great imo.
but lets not say this is the first game that has done this for me, i have being inmersed in a lot of games, Legend Of The Dragoon was a good example, like Final Fantasy VIII, but right now Fallout 3 haves no comparition for me.