Shanewatts question is a good one. I would, however, like to see it asked (and more importantly; answered) as such:
While Guardian might be the best at protecting a healer, which tank AC can form the best two-man team with a healer of each class?
I think this is an important distinction. I find that a strongly cohesive tank/healer combo can affect the outcome of games far more than a two-person team in any other MMO. My wife tends to play healers; I tend to play tanks. Our "mains" thus far have been sage/shadow and it's devastatingly effective since the two of such a pair makes up a full 25% of a WZ team compared to a smaller percentage in, say, a WoW BG. Furthermore; the tanking mechanics in this game serve to really allow a healer to be survivable when paired with a good tank in a way that dps peeling/cc and other tactics can only aspire to in other games.
What I don't know, however, is how the other tanks stack up. I feel like a tank-minded Vanguard or Guardian does not kill enough to rival what I'm capable of on my shadow but as I do not have one at 50 I cannot really say and would love to hear some other opinions. Their damage output over the course of a BG might actually be higher, but the on-demand burst a Kinetic Shadow is capable of is vital getting someone dead. I feel like my strategy is to nibble away, nibble away until I have something on the verge of defeat then burst them down before they/healers can react. It's quite effective.
How do you other tanks work with your healers? Which healers do you work best with?