If you were marooned on a desert island, it would be one of the first things you'd make (a spear likely being the other).
Props also to the aqueduct, storage tank, settling tank, and water pipes. Those are pretty important.
Props to all of the above used for waste removal.
And vaccines. Vaccines are pretty cool.
Not the wheel: We had to pretty much have flattened paths (flattened by walking) or flat areas (which we eventually moved into) before wheels became useful. (There was a great article in Discover years ago about why there were no wheeled insects (if wheels are so great, why don't bugs have 'em?) - not enough flat even surfaces.)
Not fire, because we didn't invent it, we mastered it. So we'd have to nominate the match or the crucible or the firebrick or something.