In my opinion, "best tank" is highly situational. Gallio obviously fares better against magic damage, and group stuns like Malphite work better when you have area damage to back it up. Rammus with thornmail will totally destroy a lot of AD characters. If your opponents have a fed Kat, I hope you have multiple disables on your tank...
Of course, in normal games you don't know, but it's hard to go wrong with Alistar, Rammus or Malphite.
Ali's got great support power thanks to his E. IMO, it used to be broken, now it's merely "good". He has some of the best crowd control in the universe, though it requires a bit of positioning and an aware teammate to really be awesome.
Rammus is one of the only tanks with a good chase ability, and what is easily one of the most powerful CC's in the game (taunt FTW). He also has one of the few Ults that affect towers. Like Ali, he's got two potential disables as well.
Malphite is a bit lighter on the crowd control aspect, but makes up for it with an INCREDIBLE ultimate, and the ability to burst down people at mid level and out harass people with his shield / Q He also hits hard enough to justify status as an off tank / sub carry.