The Dwarf Iron Breaker is currently the best tank in the game. It deals great damage, has great survivabilty and team mates will love your (Oathfriend) buffs.
The Empire Knight of the Blazing Sun is a good tank. Much like the Iron Breaker your team will like you for your Aura-based buffs. You don't deal as much damage as the Iron Breaker without losing some survivability, though.
The High Elf Swordmaster is a decent tank right now. You can do good damage while having decent survivability thanks to different bubbles. However, you gimp yourself with this classes' mechanic.
The Green skin Black Orc is a bad tank at the moment. Order complained that it was a tough class to take down so Mythic nerfed it to crap.
The Chaos Chosen is pretty much your only good Tank for the Destruction side. It's your Knight of the Blazing Sun for this faction. Same mechanic (the Auras) and you can do good damage or good tanking.
The Dark Elf Black Guard is probably the worst tank right now. You deal barely any damage if you're going for survivability and if you want to deal damage, you'll be made out of paper mache while still not doing that good damage.
Frankly, choose the tank you want to play, it's a 10 days trial to see if you like the game so don't play a career because it's currently a good/viable one to play. Test a career because you think like it.
-Calle