Machinist vs Summoner vs Red Mage vs Reaper

[Renathras]

Least amount of brainpower, coombos, and/or mandatory specific burst windows - Probably SMN.

SPECIFICALLY FOR DEEP DUNGEONS:

MCH has a combo and burst windows with a lot going on (Wildfire and Hypercharge with LOTS of oGCD weaves and resource management to set them up). It also doesn't have a personal mitigation or heal of any kind outside of Second Wind/Bloodbath role actions. MCH does have good personal damage and you can pool the burst for when you really need to burn things. It also has an interrupt which you can use on some things and then burst them down before they get a second chance at using those abilities...provided you're pooling your resources and big CDs to do so. It's super mobile (which can keep you from being hit but also risk you stepping on traps), but it is a little involved to play in some situations since the goal is kill before you die and you have no healing/sustain to outlast an enemy if you can't outdamage their health bar. And as you say, it's more ping dependent than the others.

RDM has its melee/cast-burst combo, but it's pretty simple to pull off. The core rotation doesn't require a LOT of brainpower, but you need to pay attention to Fire/Stone Ready procs to get the most out of it. Though not doing so isn't TERRIBLE, and the "shortcast-longcast" Dualcast system is pretty easy to do. You can super-braindead it by just going Jolt-Thunder-Jolt-Aero over and over until melee ready. While this isn't the BEST idea, you can do it if things get too hectic until you regain your senses/things calm down. The AOE rotation is likewise simple, just Thunder-Impact-Aero-Impact until melee ready, and the melee "rotation" is just one button three times. And unlike the single target, there aren't procs, so you aren't missing anything doing it that way.

Note that this isn't AT ALL optimal RDM play, but the point being, if excrement hits fans, you have a fallback that's still pretty stable. RDM also has Vercure (with Dualcast, this isn't half-bad for a self-heal in a pinch), Verraise (when playing with others), and is pretty overall straightforward to play semi-optimally (the Fire/Stone procs aren't THAT hard to deal with, and if you aren't in a party, lining up "burst" isn't really relevant other than using Manafication and Embolden when you know you can safely roll through your big melee/cast burst back to back). It sounds like it has a bit going on, and it does, but you can swap to the "simple" version and back easily, and the extra utility can help clear. RDM has long been seen as one of the best Jobs for any Deep Dungeon, including PotD and HoH.

SMN is kind of RDM-lite. Physic is sadly still worthless, but Phoenix gives you some healing, and Carby can give you a personal shield provided you use it when you don't have a Demi or Primal out. It also has a somewhat long-lasting damage buff, is highly mobile when not in Ifrit (so it kinda gets some of MCH's benefit), and it seems to do a bit more sustained damage than RDM and has pretty potent burst, especially if Bahamut is up. So it's kind of a mix of the other two. Also has a Raise for group runs. What it has over MCH and RDM in spades is that it has no combos (unless you count Ifrit's 1-2 charge/slam), no mandatory burst windows (the "burst" is just part of your rotation - Bahamut - and consists of pressing your Ruin 3 button every GCD while weaving Enkindle Bahamut, Deathflare, and 2x Festers...and that's literally it) and isn't something you have to time or anything, you just do it on CD, more or less.

SMN definitely requires the least brainpower since it doesn't have MCH's CDs and Hypercharge window nor RDM's slide-casting and melee combo stacking. It has less sustain than RDM (no Vercure) but does have a 30 sec CD personal shield that can hold 2 charges, and when Phoenix is up (once per 2 mins), Everlasting Flight and Rekindle (which, unlike Physic, DO scale off of INT and thus heal a good amount.)

Unfortunately, I don't know enough about RPR to comment on it, so I'll defer to others.
.

Easiest to hardest probably depends on if you're good at kiting and not getting touched or not. RDM is the best of the three at semi-tanking hits or making up for mistakes due to Vercure, though you need to learn to slide on the Dualcast. And if you aren't absolutely NEEDING to do the 3x melee/cast combo (where you pool resources and CDs to set up for it), the core rotation isn't bad.

.

RDM is PROBABLY the best choice of the three for Deep Dungeons (all of them) because of its on-demand self-sustain, good burst, and relatively easy rotation for "stuff hit the fan" moments. This allows it to be approachable, still do good damage so the clock isn't as much of a threat to your success, and allows some forgiveness of mistakes. Though note that RDM doesn't have a silence like MCH does (it and SMN both do have the Sleep roll action for things that are weak to Sleep, though...)

The only Job that is probably in a similar place to RDM for Deep Dungeons is WAR as it does less damage but is a lot more forgiving and tanky, and has really good self-sustain to make up for mistakes.

RDM and WAR constantly top the charts for the best Jobs for solo DD runs and they both are good in groups as well.

.

For more general content - SMN is, as others have said, an easy rotation while still having some utility and as Shiro said, some optimization.

RDM is still easy to play well in most content, though if you want to maximize it, you'd need to learn how to cram 3x melee combos in party buff/burst, though this isn't TERRIBLY difficult, and you can still clear content other than probably Ultimate not doing this, so it's more a "thing for you to work on if you want to" than "thing you must do flawlessly all the time".

So if you're talking non-solo content, SMN is a great choice as well.