I have been involved in too many Tank discussions about “ what is toughest tank” and in the end the OP picks SR, Regen, or Fire. It is maddening.
Look, how well a Tank performs against different enemies at 54x8 and tougher is largely based on the interaction with Tanks various armors.
A set like Invul that covers the classic Def/Res/Heal and can be built up is great. I have a Dark that does well, but i built the crap out of it and understand it does not operate at all like normal armors. I play it smart and it does well, for me. I have heard at least one person say it dies for them in regular content.
Even with the toughest Tank built near perfect once the enemies are dialed up you have to be proactive. So, once you listen to all the advice and pick a Fire armor I wish you great luck. Just play actively and play to win.