If we're talking about balance within 8-man endgame content, they're all similar in terms of DPS. In terms of defense, there's more variation based on utilities, CD lengths of invulnerabilities, and on-demand mitigation, but it's still pretty balanced and at that point the tanks are more a question of what you prefer in terms of gameplay style and mitigation flavor.

If dungeons are more your thing, WAR and DRK are the best, while GNB flat out sucks! All other tanks can grant themselves some degree of independence from healer intervention, but GNB just BEGS healers for their GCD healing. Now, if you're a skilled tank and understand how to utilize and space out your cooldowns, you might respond that you've been "doing just fine," but it's something that would be A LOT more fine if the same was done on literally any other tank job.