I havent played all classes enough to know which is most boring (WHM might be a strong contender) but the differences between WAR and PLD is Clear. Although Rage of Halone and Circle of Scorn look flashy, Doing the same rotation gets boring. I think PLD downfall is that It's a simple class to play. Not many Choices of what to do.

Single ? = RoH
AoE ?= Flash
Big Damage incoming ? = Pop a Defensive Cool Down.

WAR on the other hand has to alternate between 1 of their 3 Combo's Depending on situation, Manage Wrath, and make decision on the fly on which ability to use their Wrath on. All while doing the same things PLD has to do above .