I have all three tanks done but WAR probably the easiest to start with if getting into the tank role. War feels like a DPS kinda is why as it gets a very strong aoe that does decent dmg and gains hate and it's available at low level.

As for pure tanking? I think PLD is the best when mitigating damage when it gets to level 60 and up.

DRK was my least favorite tanking job to level it felt more complicated to play compared to WAR/PLD. Only reason I bothered was for the cool glamour.