I've got a 41 Willpower/Shovel tank on the go at the moment, and she's great fun. Had a 37 an Inv/SS one that could tank +12 Maltas (tanked a +4 Battle Maiden and didn't get hit) but couldn't do any damage (concept reasons), and I'll definitely be making a Dark/Dark tank (I'm a bigger fan of Dark).
I've also tried Stone/Stone, too, but gave up at 14. I've been tempted to try Fires and Ices, too, but never got around to it.
The main thing is, though, they don't "play" the same. Sure, they're all tanks, but I have to change the way I play them to make them useful. Same with all my 'fenders. As the sets aren't carbon copies with different names, there's plenty of reason to "make another AT", regardless of which AT that is.
But, yea. Concept comes first for me.
Currently I have two tanks. Each I've had a lot of fun with so far.
From a personal point of view...
My tanks...
My Invulnerability tanker is my Superman concept character (with additional powersets according to his bio). Becuase the look of the set doesn't hinder with the look of your character it is probably my favourite out of the tanker primaries.
He's a fantastic scranker despite only taking 3 real attacks from the SS Pool (Haymaker, Knockout Blow and Footstomp).
The Invul powerset, although not THE strongest armour, is a renaissance tank and will always usually be one of the favourites for various TFs, Trials and missions (subject to arguement: stone...too slow ; ice or fire...survivability lacking; WP...doesn't hold aggro as well ...etc etc)
My Ice tanker is fantastic for survivability...but only when with the right(/experienced) team!
This is one hell of an aggro magnet. A totally different style of play compared to my Invul tanker, but equally as fun.
Our SG team has both Ice (aggro magnet) and WP (survivability) tanker and with the right communication they're able to compliment each others build during missions making it very, VERY safe for all involved in the team.
Other sets tried but didn't last...
Stone is just too ugly for me and it's hard for your character to show when incased in the very generic granite armour.
Fire is something that I've always contemplated merely for it's offensive purposes.
Not yet tried as a tank...
Willpower...I've not had much interaction with playing this set (apart from a lvl17 scrapper tht I don't regularly play). I already have a Regen scrapper that works in the same principle. Probably something to consider a bit later.
[edit] Scarey...each post was made an hour from each other ^__^
@Captain Solaris
Guild of Extreme Heroes "Strength is in Unity"
I don't think different tanks for different situations I just think different tanks, different limitations, different flexibilities -> new ways to go about things.
Some people may think that the only Tank thats worth anything is one that can have people shoot fish in a barrel but I much rather a case of people not being good because of how they build but how they and the rest of the team move. It is largely for me about having to think. Maybe not just you but the people around you too. If you build a super tough tank thats virtually unkillable that gains all aggro who has to think? (I am thinking about doing one just to say look there it is, look now I can do what you do 
) .
Each type of Tank may have to face different enemies in different ways and people playing with the different types may have to adapt. Once you put one primary with another secondary you are different to someone with the same primary with another secondary.
Some people have different types of Tankers for PvP as well. I am global friends with one who does (despite trolling pvpers incessantly I do have some as friends and some admit to being my friend 
).
You could be I love tanking so much but I wish "these were easier" or "I am sick of teams that do such and such" and make a tank where that doesn't effect you.
I got a few but I got a few of everything. I think you can learn a lot about tanking from other people and from even playing different characters. In playing defender type A I realize what I should of done when tanking for defender type A, later when another defender type A is not doing what I would do as one. I have to consider it payback for me not tanking right for the first defender A...but really to suffer the pain and agony of playing through things again I would need to have a concept.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
I currently have 5 tanks atm 2 50s 2 mid 40s and a lowbie in the 20s. My 2 50s are stone and ice and my 40s are inv and fire my 20 one is wp and i find that all of them play differently. There are some situations that I find I would prefer another tank but I havn't found anything yet that couldnt be tanked with any of my characters.
@Mister
Mister Volcano lvl50 Tank SA/FM
Silver Sight lvl 50 Controller Mind/Kin
Cryogenic lvl 50 Tank Ice/Stone
Winters Night lvl 50 Corrupter Ice/Dark
Chaos Agent lvl50 Brute EM/EA
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It will most likely be a ice/dark tank.
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Definitely rolling one of them here 
Zomg! but but.. if you play a scrapper that hides beind the defender... I dare not figure how you will be playing your tank!
[ QUOTE ]
Zomg! but but.. if you play a scrapper that hides beind the defender... I dare not figure how you will be playing your tank!
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That is Blasphemy!
Both DM/SR and DM/Regen are stupidly good against AVs once ToF is stacked so I cant see why it wouldn't work with any tank set at all.
As to why different sorts of tanks....... I'm the Ice tank in capts SG team. Ice gives me one hell of a buzz to play, and grabbing agro is a dream with it. It feels completely different to my WP/Fire tank where all I have to worry about it grabbing agro. Its nice to play both sides of that as when you're afk in PI in a group of +7s easily enough but cant keep the agro off a well built scrapper........ well its not at all like playing an Ice tank.
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Some interesting responses. I personally love having different toons for different things. Ie for TFs, generally my illu/rad gets used, and only my tank if its him im doing the tf for, or the team needs a tank. My domi gets used for LGTF a lot, my stalker for speed runs ect. In terms of AV killing, he may not pack that much damage, but he can generally survive, though I'de probs want a backup tank for tanking Lord Recluse if I ever try it. While getting aggro and keeping it with him can be a problem sometimes, its gotten a lot easier since I 6 slotted taunt with a 5 of the perfect zinger set and a rch.
But still, an ice/dark tank has an appeal, but then i have a thing for dark paired with defence sets (DM/SR stalker and scrapper, bubbles/dark defender). Only thing is, at lower levels, WP for me is great and suffers very little. At high end, he is just nuts to play. Just makes me wonder if after loving my first tank, whether ide be able to give another tank, the same love.
I've nearly got my WP/Mace tank to 50, and have enjoyed him quite a lot. I've enjoyed him so much, that I thought with the new tank sets coming out, a ?/dark melee tank (big fan of dark) might be in order. It will most likely be a ice/dark tank. But really, If I have a tank capeable of taking on most AVs, especially with a bit of backup, is there much point in rolling another tank, other than to just have another tank to lvl? I know some people have a few tanks, and would like to know if they use different ones for different things, or why they have more than the one tank.