Which Tank is the most mechanically challenging?

[Sazuzaki]

PLD and DRK are the most mech challening IMO. PLD def has the most complex rotation, very hard to use it to it's fullest. DRK on the other hand has a very high defensive skill ceiling. You lose DPS if you use TBN (The Blackest Night) improperly cause it costs MP. PLD for hard rotation management, DRK for hard resource management.

Both have downsides, however:
PLD is very spammy with Requiescat, Atonements, and the new Confiteor "combo." So you're gonna be pressing one button a lot with PLD.
DRK, you only spam 1-2-3 outside of burst windows which is really boring, as well as our inferior Inner Release called Delirium. A lot of its kit is very unimaginative and lacks synergy. Gonna be real, the only reason I even touch this class is TBN.

Honestly though, there is no real hard role in this game. All of them can be played by anyone, which isn't a bad thing. The real difficulty comes from the fights themselves.