Played BLM, SMN, and NIN at 60, in order of easiest to hardest, I would say

BLM --> SMN --> NIN.

BLM is very easy because it's the same rotation basically every time. You can do very high damage with just a few buttons.
SMN is very easy until 60 when it comes a little more difficult because managing stacks to do Dreadwyrm trance. Then it became a little bit harder.
NIN is the hardest for me, but the most fun, because I need to watch a lot of things. There are many Abilities you can use in between Weaponskills and watching the cooldowns on all of them is important.
When is my Jugulate available? How about Mug?
When should I use my trick attack? How much longer before I can use it?
When is Dream within a Dream available? How about Duality? I need to Refresh Huton, should I use Duality with Armor Crush or save it and use it with Aeolian Edge in the next rotation?
Ninjutsu is available, should I use Raiton or Katon here? Or should I use Huton because I won't be able to make use Armor crush in time to refresh it.

And you have to think about positioning too. If you can't get behind the mob, should you still use Aeolian Edge? Or use Armor Crush? Is there enough leeway to get to the flank?

Being sure to use ninjutsu, jugulate, mug, dream within a dream everytime they are available and constantly keeping huton up and using trick attack as much as possible is far more challenging than BLM for me.
Playing BLM I felt like "Find a safe spot to stand. Ley Lines, Enochain, Fires and Blizzards as long as possible. Repeat".