Melee: RPR
It's opener is easy and it follows a priority system that is EXTREMELY easy to follow, plus is generally slow on the button presses outside of shroud.

Caster: SMN
No brainer...

Ranged: MCH
The rotation is VERY easy 1-2-3 - BIG HIT - 1-2-3 - BIG HIT - HYPERDOUCHE - 1-2-3

Do not go near the following though:
MNK:
Easy until you want to actually play it optimally

BLM:
Aint played it but heard you really need to know the fight to do well.

That is my take anyway.