Literally depends on what the player does.
I've had tanks of all 3 jobs who by my eye seemingly did nothing yet took near 0 damage so i could basically cleric and ignore them.
and then I've had people on all 3 jobs doing lots of work, dodging and cycling CD's etc but taking a buttload of damage for no reason.
Couple this with whether the tank utalises their own healing set up or not marks a wide variable on "Hardest" to heal.
that and i guess your own mind on what constitutes hard healing.
going by experiences, id personally say PLD, followed by DRK, with WAR being easiest for me