All the tanks are completely fine, to address the op. I personally feel that drk shines in 5 (just get good at plunge timing), 6, and god kefka. Progressing in god kefka with pld/war, it was pretty obvious that drk would've had it good in that fight with tbn and 60 second dark mind. There's so much damage going out in that fight almost all the time (auto attacks crit for 25k by the way), so having that tbn every 15 seconds would be really really nice. It also alleviates your cd management throughout the fight.