Tanks are a very important part of the game and a type of hero you will definite consider for your team. Some teams don't have any tanks, but those expose your second line and are very risky.
The first tank I'm going to talk about is Galahad. Galahad is a great tank to use. He does a lot of damage he is the first hero you get in the game. His grey skill, Iron Skies, brings down swords from the air that strike your opponents and deal AOE damage. His violet skill, Harun's Pride increases that damage even more with pure damage.
Another great tank is Astaroth. He is the normally the second tank you get. He can create a shield with his grey skill, blocking physical damage. His blue skill, Demon's Allegiance, makes Astaroth maintain a link with the hero that has the least health or is closest to him. Whenever that hero takes damage, Astaroth takes some of it. Why so good? Because he lessens the damage dealt. His violet skill, Last Word recarcinates himself or another ally once per battle.
The third tank I will talk about is Aurora. In mobile, you get her near the beginning of the game, but in browser, if you don't get her some other way, you'll get her when you 3-star Chapter 7. She is great because her grey skill, Crystal of Selias sends a blast of light from the ground dealing magic damage. Her other skills stun, deal damage, and her violet skill, Rainbow Halo, blocks out some magic damage and after a while, the shield explodes dealing damage to nearby enemies. Aurora's artifact also allows her to dodge so that makes her a great tank. When patroned by a Violet or above Cain, Aurora will continuously gain energy, and will use her grey skill, which activates her artifact. This will boost her and the rest of the team's dodge even more, allowing her to gain even more energy. Aurora is easily the best tank in the game.
Those are just a few tanks to upgrade and another article will be posted soon for more tanks to level including: the rare Cleaver, the undead damage dealer Corvus and the taunter Ziri.