NIN, SAM, or RPR

[Khalithar]

I'd say reaper is easier personally. Your positionals aren't part of your base combo and your base combo is a simple 1-2-3 button press. There's a little more when you unlock demon mode (and it's faster than the normal GCD) but it's not something you have to deal with constantly.

The only thing I don't like about reaper is having to maintain the debuff. The maintenance buffs for sam are a lot more fun to manage because as long as you're doing your combo correctly you maintain them, whenever I have to refresh my debuff on rpr I *always* feel like it's interrupting my rhythm.

[Caldra]

I have all 3 jobs at 90 and have played them all to some extent and I can confirm, that reaper is the easiest to play for reasons, that have already been said.
Between Sam and Ninja it depends on what is harder for you to do!

Ninja has a very busy burst phase! You have to hit all of your skills in your burst window to maximize dmg! Outside of your burst window, ninja is fairly boring, as you're only spamming 1-2-3 combo!

Samurai, my current main, has a busy burst phase, but not quite as busy as ninja, but also two 1-2-3 combos and one 1-2 combo, that you have to use correctly!

Some people think, that fitting all spells in your burst phase is harder, some think the alternating 1-2-3 combos are harder!

So you have to see for yourself, what is harder for you!