Heres an interesting rundown of some of the top underground transit systems in the world, with some interesting facts (like every building in the city of Paris being within 500 metres of an underground station) about the networks.
London is of course voted number one. I have travelled on it many times, and think it is superb. The only improvement would be air conditioning. Did you know that there are fake houses covering a tube line in Westmister? 23-24 Leinster Gardens were replaced by a facade to allow trains to vent.