Easiest: DRK
Mild: PLD
Hardest: WAR

As for "Fun"
PLD and DRK.
PLD is a bit more of a challenge, but still plenty easy.
DRK is straightforward. (I actually find DRK boring as heck, but there's quite a few people who find the simplicity of DRK fun.)

PLD is the best in pretty much any situation, be it MT, OT, ease, fun, etc.

The only exception is lower levels, where maybe WAR is more fun, since it has berserk, and PLD lacks the fun abilities till closer to 50+