Muradin. The Tank new accounts can pick as their first Hero, and the only starter Hero not given by Spoils of War. As such, if you own Spoils of War, you should pick Muradin, even if you're not interested in playing him. Anyway, never out of the fight for long (until he runs out of Mana, anyway), Muradin is good at interfering with enemy pursuits and escape attempts while not dying himself, and can even execute lone Heroes by himself pretty readily.

Diablo. The Tank from the opening cinema, and the highest HP Hero in the game when you account for his Souls-derived HP. A versatile Tank who can build to absorb tons of punishment, to safely poke victims continuously while he waits for a good opportunity to jump in, or to all-in on charging in and ruining victims foolish enough to stand near walls. Also has some of the best positional control in the game, which is very powerful when eg fighting over a Mercenary Camp's Capture Point.
Anub'arak. The last Spoils of War-provided Hero, and a Tank whose design probably made more sense for the environment he was released in than for current conditions. Bizarrely good at split-pushing and the taking of Mercenary Camps, and extremely effective at disrupting enemy plans.
Cho. The physical aspect of Cho'Gall's unique two-in-one design. More mobile than you might expect, able to survive a lot of punishment, and with decent damage in his own right.
Gall. The blaster caster portion of Cho'Gall's two-in-one design. Damage output that's almost impossible to suppress, and which doesn't disrupt Cho's activities, making the pair one of the more relentlessly deadly pursuers in the game. Also gives them a bunch of truly unique advantages, and, well, some unique disadvantages. I'd also argue Gall is one of the better Heroes to potentially try to ease a newbie into the game with, since Gall doesn't have to concern himself so much with positioning and map awareness and all, and in fact is liable to practice map awareness just from getting bored during travel and whatnot.
Blaze. A Tank with excellent waveclear, very good ability to interfere with escapes and pursuits, and good damage output in general, but an intense hunger for Mana. Alternatively: the Tank I'm always left confused as to why he's not classed as a Bruiser.
Tyrael. An atypical Tank with high mobility and the ability to directly shield allies, but heavily reliant on body-blocking and a specific Heroic choice for interdicting enemies.
Arthas. A low-mobility Tank focused on hindering enemy mobility and weakening Basic Attack-focused enemies, with strong self-sustain and better damage than people tend to expect.
Johanna. A Tank who can shrug off attempts to disable her, shut off Basic Attack-focused enemies, disrupt enemy plans, and with better waveclear than most Tanks, all while being extremely difficult to actually kill. Poor ability to push damage onto enemy Heroes, though.
Stitches. A Tank with strong self-healing and the ability to pull enemies out of position from quite far away.
Garrosh. A deceptive Tank who gets harder to kill the closer to death he is, able to throw enemies into bad positions if his enemies give him the chance to be in arm's reach for a moment, or hurl allies to better positions to potentially save them. (Or enable their pursuit, or just use them as ammunition to finish a fleeing foe)
Elite Tauren Chieftain. (Always shortened to 'E.T.C.') A highly disruptive Tank with strong ability to shrug off sustained punishment, interfere with pursuits and escapes, and okay damage output. Both Heroics are very different gamechangers.
Mal'ganis. A Tank who has incredible ability to keep going in the face of sustained damage, can outright punish focusing him (To an even greater extent than the usual 'focusing the Tank is probably a mistake' axiom), and can be quite disruptive. This includes that he has access to one of the best options in the game for nearly-continuously Stunning enemies, though it relies on his foes being predictable... such as because a Boss Mercenary Camp is being fought over.
Mei. A rare ranged Tank, with significant capacity to keep enemies Slowed and otherwise interfere with their efforts to go where they want, and good self-sustain. Mana-hungry, and easily able to accidentally save enemies with her Heroics if they're used poorly.