If you wanna be a real heavy TANK and nothing but a TANK, play a Guard.
IF you like to DAMAGE really heard, play a DT.
IF you love to tweak and study, to be most loved by your group and be settled for ALL Situations, chose a Conqueror.

A Guardian can be extremely useful as a Tank also in early stages with lower gear.
A DT shows his power earliest, when he is 80.
A Conq needs Gear, his power comes with T2 or comparable epic Kithai.

The most variable class is definately the Conq. He can be both: a damage beast and a Tank. And he is the only one in the entire game who can rezz while battle. Conq is the Kings Class of the Soldiers, if not overall in the game.