Nowadays there isn't really any synergy between tanks that would make or break a fight, really any combination works.

You could look at what damage buffs your party possesses since WAR for example doesn't gain much from crit or direct hit buffs, or go with GnB if you want the two tanks with the current highest dps, but neither of those are necessary to beat the fights either.

As for who should pull the boss? Basically anyone except the DRK, since pulling with Unmend messes with your opener.