The Tank or Heavy is generally put in the First or Point position of a line-up. It's JOB is to get attacked, so the rest of your cards, don't.
THE absolute, hands-down, #1 Tank in the game is: Lord Arianthus
He's Neutral, so you can use him in any deck.
He starts with 6 Health but he gets 8 at L3 and 9 at L4, which, still- isn't much,, for a Tank. The big deal with him is his abilities- he starts with Shield AND Void, so he takes one less damage from each attack. Monsters with an Attack value of one (1), CAN'T HURT HIM!
Then, with L3 and 4 he gets Reflect and Thorns so when he's hit with Magic or Melee attacks- he damages his attacker, whether he took damage or not.
He's also, relatively cheap, in mana compared to most Tanks- 5
Now, he's PRICEY- whether you're looking to buy him or rent him, which makes sense as he is the best. So, what is it that makes him 'The Best', exactly?
He's a Neutral- which let's you play him in any deck.
He has 'damage reducing' abilities- so he can absorb more hits (Shield, Void)
He has a good amount of Health- again, so he can absorb more hits sparing the rest of your team damage.
Also, he can (at higher levels) deal some damage to the other team- which is a bonus.
Neutral- Some other, good, Neutral Tanks:
Chain Golem: Not a Lord Arianthus but a VERY good substitute. He only starts with Shield but he has a higher Health total (11) and he gets a point of Health and Void at L2 and instead of gaining Reflect and Thorns, he has 3 Melee attack. He costs 8 mana instead of 5 but he's significantly cheaper to buy or rent- he's also Untamed instead of Beta.
Goblin Mech: Has pretty high health (11 between Armor and Health- rising to 13 at L3) and decent Speed (3) which will allow him to Dodge some attacks. He's also cheap to buy or rent but costs 10 mana.
Cornealus: 12 Health + Heal should allow him to absorb multiple hits over a couple of turns, he's really better as an Anchor as he has Ranged Attack, not Melee or Magic. He is expensive though, both to buy/rent or to play (10 mana).
Gelatinous Cube: 8 Health for 6 mana, (gaining one every level, until L6- when it gains Heal) has Scavenge which let's him gain Health when monsters die during the battle. No Attack and no other abilities but he's SUPER cheap to buy or rent.
Water Water is STACKED with Tanks- from the top to the bottom!
The Kraken: Probably the best Tank in the game but only available in the Water Splinter- this monster is 12 mana with a 3/14 Armor/Health stat-line. He also come with Taunt making him impossible to ignore. To make matter worse- at L2 he gains Demoralize, making him one of the most frustrating monsters to play against while making him indispensable in all high mana Water matches.
Phantom of the Abyss: While her 7 Health for 10 mana is not impressive and she probably makes a better Anchor at the back of the line-up with her Magic Attack, her combination of Dodge, Flying and 6 Speed means that she is virtually impossible to hit by any slow, land-lubbing monsters.
Torhilo the Frozen: 9 mana with 4/8 in Armor and Health, this monster also starts with Void- meaning he can take some hits before going down! Also packing 4 in Melee Attack- this means he can dish out some damage before he falls.
Sea Monster: Classic Tank- 8 Health w/Heal to start. This Common card can man Point for a couple of turns before falling while his 4 Melee gives him the ability to dish out some damage himself! Gaining a point of Health at L3 means he can heal 3 points/turn while the addition of Thorns at L6 just makes him vicious.
Frost Giant: 7 mana, 9 Health makes him efficient and meaty! add in Shield and 4 Melee attack and he makes a great Tank at L1. When he gets L2, he's got 5 Attack and Trample- which lets him attack more than one opponent in a turn.
Lobstradamus: an unusual choice for a Tank but with his 9 Health for 6 mana plus having Shield, he makes a great and very mana efficient choice. Level him up a little and at L3 he has 11 Health and Reflect so the attacks he doesn't reduce, he sends back at the attacker.
Spineback Turtle: 4 mana with 2/6 Armor/Health- he's super efficient from the get-go (F2P). It's when you level him up that he really shines, though. He gains Health or Armor each level until L4- when he gets Thorns. At L4 with a 3/7 Armor/Health stat-line and Thorns, He just can't be beat as a low cost Tank.
Earth This whole Splinter IS Tanks! High Health/Armor and protective Abilities everywhere.
Kron the Undying: 10 mana/10 health, Healing- at L2 he gets Divine Shield and at L3 he gets Last Stand. He, truly, lives up to his name. Also, PRICEY- both to rent and buy.
Flesh Golem: 6 mana/9 Health- at L3 he gets Healing. Budget Kron- in both DEC and mana.
Stone Golem: 5 mana/8 points in combined Armor/Health (2/6). He starts with Shield which reduces damage from both Ranged and Melee attacks- makes a fantastic budget Tank.
Biceratops: 4 mana/5 points in combines Armor/Health (1/4)- he doesn't start out as a very impressive Tank but as an Untamed Common who gets a point of Health and a point of Armor by L3- he makes a surprisingly good Tank for the cost.
Light Another, good, Splinter for Tanks- lots of Armor and Health on these guys!
Kralus: 12 mana/17 points in combined Armor/Health (5/12)- this cat is rocking both Flying and Immunity from the start and gains Healing at L2!
Shieldbearer: 8 mana/13 points in combined Armor/Health- He gains a point of Health at L2 and he has Taunt, meaning every opponent who CAN attack him, MUST. Whether you put him in the front or in the back- the Shieldbearer is one of the best cards to divert attacks away from the rest of your team.
Silvershield Paladin: 5 mana. Starts with Shield and at L2 has 2 Armor and 6 Health.
Crystal Jaguar: 4 mana/6 points in combined Armor/Health (1/5)- gains a point of Health at L3, Thorns at L4.
The last 2 splinters- Dragon and Death, just stay away from until you've been in Gold for a while.