When you first ding 80 and start gearing up for raids, it's easy to forget that you're not going to be tanking them alone. Yes, the odd encounter in Naxxramas will only call for a single tank (as will Malygos) but, for the most part, you're going to be in there with another heavily armoured rude-dude who will be taking blows with you. The old terms of "main tank" and "off tank" have been dispensed with by many people in an attempt at political correction in an online sense, but the old titles still work because one of you is going to be tanking the boss and the other is not. And let's be clear; for a successful raid, this person shouldn't be considered "the other guy".
He should be considered a part of your very own tanking repertoire.
Now, the guild often asks who the best tanks are, but they do so on a class to class basis. The truth, however, is that class doesn't make an awful lot of difference when you're going to fight the most dangerous bosses that Azeroth has to offer. What matters more is how the tanking team understands one another, exhibiting that understanding throughout the encounter and seeing a plan through to fruition. It's often said that tanks are not the most important part of a raid, but to the more erudite they most certainly are. That is not to say it's the hardest job (it invariably isn't), but the tanks carry the responsibility of the entire raid on their shoulders and the entire battle plan hinges on their actions; the rest of your raid will react to how you act, basically. This means that your tanking team has to be as good as it can be, in every way, before you pull.
Enter, Thorian: accept no substitutes.
It just so happens that our raids are composed of a warrior main tank and a paladin off tank, often considered one of the best pair ups. In saying that, this has less to do with Devastate and Avenger's Shield and more to do with Zellviren and Thorian as people.
Let me explain.
Since roughly the beginning of Wrath of the Lich King, Thorian and I have been the guild tanks. We've slotted in the odd warrior, paladin, druid or death knight for certain evenings, but there is no doubt throughout the whole set up who the main men are. This partnership has been deliberately cultivated and it's one that is now, quite frankly, demanded by the rest of the raid group when we're pushing content. The reasons for this are quite numerous and I'll elucidate them for you here, in an effort to point out how you should best form your guild tanking team.
1) I enjoy conventional "main tanking" and Thorian enjoys conventional "off tanking".
2) We complement each other nicely in playstyle and spec.
3) There is an understanding between us of what is expected generally, and specifically (considering encounter).
4) The rest of the raid trusts us, and we trust each other.
From the points above, you should note that points 2, 3 and 4 can be achieved through discussion and, above all, experience. The most important point, however, is point 1 - we both do what we actually like doing. There are occasions when we switch around for a bit of variety (and it keeps us both honest) but, when push comes to shove, we both like the roles we fulfill and wouldn't want to change them.
I would suggest that is the most important thing to consider when fixing your tanking team.
Thorian and I got that by a bit of luck and, in many cases, that's what you'll need. But if your guild runs several tanks, you're best assigning someone a role that they enjoy most, as you'll get the most out of them by doing so. Whether the tank is a warrior, druid, paladin or death knight, they are all equipped with tools that allow them to do any tanking job.
Make use of preference.
You won't regret it.