As it stands Inv is probably the best mobile tank (until if they ever decide to 'fix' the def buff from invinc which is about 2-3 times what it should be according to the devs).
Stone is the toughest by a mile, but don't underestimate the slow and rooted effects, and only once it hits 32+ in level. I have a 50 Inv/ss tank and a 39 Stone/Stone tank, simply not being able to run around whilst tanking and even get over a small decorative ledge without the use of teleport is pretty huge.
Secondary wise, well Fire is good for AoE, stone is entertaining and reasonable with its big hits, SS is an all around good clobbering set but energy probably wins out overall in single target damage (again probably late game only, takes a bit of getting going).
As I say though Primary wise stone is toughest by a long shot, you just need good patience to be able to do much with it, and don't expect it to be that much fun in PvP unless you like just being a wall. I've stood toe to toe with several stone tanks with my blaster and they don't exactly cause me much panic in the damage they kick out unlike some of the other primarys can not just in damage output but mobility in being able to chase you down.
This is how I view the Secondaries:
At high level (35+) Energy is the winner for single target damage, the two high level attacks it has are also mostly Energy damage which is less resisted (the other attacks are mostly Smashing). Good Stun effects as well.
Fire has by far the highest AOE damage and good single target. No secondary effects.
Ice lowest damage, best set of controller-like powers: -SPD, -Recharge, Hold, AOE Knockdown even a Sleep!
Stone High single target damage, a nice AOE Stun, very entertaining to play.
Super Strength attacks are very similar to Stone but on the whole not as fun to play. At high level probably has the edge over Stone because of Rage (which is like a nearly permanent Build Up) and higher damage AOE attack in Foot Stomp.
My very good friend (Amducious) and I, have been pondering over the best builds/power sets for archetypes. I'm no expert on tanks, that's his job, he has a lvl 50 inv/ss under his belt. He thinks that Invulnerability is the best Primary, but is not sure on the best attack. He explained to me his view on some of them, and what they're good for, but what's everyone else's opinion and why?