The most tanky will almost always be Cho'gath since the time requirement for his stacks is a lot lower than Sion's, as for which the best tank is right now of those three, hard to say but probably Galio? He definitely has the highest damage output of them and can lane relatively well. The problem is that you're not very useful in teamfights outside of creating space and ulting on the engage/counter engage. But I think he might be the best pick up for normal games/lower ranked games. He is for sure the least tanky of the three though with how you build him.

If you don't like that then yeah I'd say Cho'gath, Sion's laning is very easy to abuse and if you learn to play Cho'gath well and use your Righteous Glory well, your ulti can make a massive difference in a teamfight.