Morgana is a superb tank. She probably your go-to tank for a poke team.

Quit assuming tank means "beefy melee that doesn't afraid of anything"

Tank starts fights, gets in the way, usually has some sort of CC, isn't ignorable.

Soul shroud into whatever you want. She has good enough base damage that she doesn't need AP for most cases(though pure AP morg does shred). Her prison is one of the best initiation spells out there. If you've ever been in a mexican standoff at a turret you know that the morg player's team wins almost every time. Her ult stuns everyone she gets near. And she can control positioning of the other team with her aoe.

I'm surprised nobody has mentioned malphite. In ranked hes a very reliable pick, which means hes great to learn early. Nohing worse than playing amumu all the way to 30 and finding out you can never play him again due to bans.

Amumu's my favorite tank, but I don't think he's the best initiator in the game. Malphite is usually the one given that title. Amumu can initiate well, but it's tough to position him in the center of the enemy team and snare them all. You can bandage toss in, but that'll usually get you to the outer edge of the enemy and you might only get 2 or 3 of them in his ult; not to mention that you won't have the utility of stunning during the team fight or chasing after. Ghosting in is nice and it causes the enemy to scatter, but again that's good and bad: forces them out of position, and possibly, also out of your ult. Flash range sucks.

But if Amumu can get some help initiating (say a bait or a Morgana/Ryze snare), then he can just walk in and get the entire enemy team with ease without having to waste any spells/stun.

Gragas!

Rammus in my opinion is the best tank, my favorite to play by far.

His powerball speeds him up pretty well, and does damage and slows when you make contact. My favorite thing to do is have flash so that you can activate your power ball behind a wall of minions, and your enemies wont think a thing of it. Thats when you flash past the minions and hit them. always fun =D

His shell ability increases his armor by a ton (at max level increases armor by 100) and returns 20% damage done to him in magic damage

His taunt is amazing, it taunts people for a couple of seconds allowing your team to pounce on them and reduces their armor significantly

His ult is great as well, he shakes the ground around him and deals some pretty decent damage in waves for a few seconds, its an amazing DPS and very useful in a team fight. He is able to move around while doing this, it also damages towers and inhibitors.

His passive is awesome for a tank, it converts 25% (i think) of his armor into attack damage. He is by far my favorite tank to play, i have the most fun with him

In high ELO gameplay Alistar is the best Tank. He is dam near invincible, and can absolutely tank any turret alone.

contrary to popular belief, Galio is not suppose to be the main tank. He is the guy that goes in after the tank and soaks up all the damage with his god-game breaking ult. If forced to tank though, I would say he is better then rammus. He can get the whole team to focus him instead of just one person. His shield adds a whopping 90 mr and armor to whoever he cast it on. he has higher burst damage. Rammus is better at general survivability with his DBC and Powerball though. Galio is also better during the laning phase IMO.

No nasus mentions, I am disappoint.

My Favourite is Mordekaiser, you can tank and get much frags too.