I voted for Fallout. I love Planescape, but at the end of the day, it is actually rather linear. It is a lot of fun to explore it, mind you, and it does world building really well, being matched only by Ultima before it. However, the problem is that it is limited by its own focus on its story. It is more concerned in presenting a cool story and setting to the player than letting him interact with it.
Fallout, Fallout 2 and Arcanum, on the other hand, present the player with a real interactive world. One where your decisions do have an amount of consequence. There is a lot of care in making sure there is more than one way to approach each problem and the player is rewarded for paying attention to the world and attempting different things. Of these, I would say Fallout 2 was a bit weaker than the others, for while it inherited a lot of design from Fallout 1, it seemed a bit watered down, with a lesser focus on giving the player options just like the 50s science fiction setting of the original was a bit watered down as well.
Arcanum probably had the best world building of them all, with some quests being approachable from basically anywhere. There was, I think, a greater emphasis in making the world seem real and interactive, rather than simply the backdrop of a story. Unfortunately, it also had several flaws... I don't even mean (just) the combat. The whole game system was a bit clunky and several of the possible paths for the main character just don't really work. So, while I love Arcanum, I think all of its flaws do end up pulling it down a little... or a lot.
Fallout has lots of flaws as well, of course. But at the end of the day, it is still a lot of fun to play. There are lots of things to find in the game, a whole lot of great characterization, the focus on the old science fiction is spot on without being tongue in cheek and, in one of its most balsy moves, it actually makes time an important resource. So, my vote goes to Fallout.