Dark Armor is the heavyweight champion. The best of the best of armor sets available for a tanker. It requires investment and knowledge but can be made into the ultimate force. I love my Rad tanker also but he is the silver medalist. Dark Regeneration's 30 second base heal vs 60 for Rad therapy and more importantly DR's healing does not drop off where as Rad's heal does after the first target to 25 base per target. Even factoring in particle shielding the combination of both still does not exceed the potential benefit of Dark Regeneration and one click. A second benefit of Dark is the resist set bonuses are paired up like Smashing and Lethal, Fire and Cold, Energy and NE, Toxic and Psy. Dark does not need to chase 6 slots down on sets. I tend to ignore Toxic, the least seen damage type and Psy is already capped so you get to slot 5 on quite a few purple sets freeing up for proc's. Psy damage is everywhere and not needing to do anything at all to cap it is too good.
All that is not even factoring in Cloak of Darkness. I love to run as many defensive toggles as possible. Combat jumping, Maneuvers, Weave typically with hasten. With Dark Armor you can run 4 defensive toggles and have hasten. What that does is making picking sets bonuses far easier since hitting the soft cap in S+L and melee is quite easy.
Cardiac is Dark's best friend. The 20% bonus to resist powers adds up to about 4% total resist and slotting the resist ATO in death shroud almost guarentees's multiple procs adding up to over 13% resist. Even with weak base energy resist (20) a DA can go 90 to all resists except Toxic. Soft cap defense easily and have the best heal in the game with soft control CC.