By and large, the 'easiest' DPS rotation is probably DRG. It's a follow the flowchart 1-2-3-4-5-6-7-8-9 rotation. That is a gross exaggeration but the base non-oGCD pattern is very static with zero flexibility that isn't super fight specific nitpicky and also ignores a lot of the more strict timings involved with it (oh no you delayed a jump you are now setting yourself back forever in your rotation). That said, you'd find many people will make that same argument for every melee since at the core every rotation is just that - a static rotation. Monk gets a special consideration, at least in my opinion anyway, since the only way to be competitive at Monk nowadays means doing TK openers which are wholly divorced from how basically you play Monk normally (aka, Fist of Wind based TK / double TK openers). It's still not "hard" and it's a case of muscle memory but it's at least something you haven't been doing for most of your leveling process which makes it a suddenly new thing. And then NINs can make the argument with TCJ giving them a timing reset in their opener and---
--and that can go on for awhile. Basically, there isn't necessarily going to be an easiest that doesn't boil down to a core point of practicing the rotation. Learn it in chunks. Isolate parts of it. For Monk, it's get your basic triplets in order since the timers of DK, Twin Snakes, and Demolish lets you be flexible; then you can start working in keeping your oGCDs lined up properly / Riddle of Fire windows; then you work in your TK opener; last thing to work on is min/maxing your GCDs to learn how to TK under every Riddle of Fire and Riddle of Wind + PB your stacks back. Your goal is to get as much of the rotation known to the point that you're not constantly thinking "and next comes...?" I mentioned it in a previous post that was probably 8,000 characters too long but
XIV Analysis is really good at getting your consistency down with the basic rotation aspects. Don't worry about the finicky things, start with the biggest pieces and move on down to the specifics. Know your GCDs. Know your timers. Know your cooldowns. Then you can work on knowing your rotation specific tricks to get more out of your DPS. 50th percentile isn't about being flashy. It isn't, even at this point, about gear. It's about having your basics down.
Ninja has maybe one big 'exception' to the above in that doing your first Ninjutsu correctly, or more specifically delaying your Ten Chi Jin* a few seconds, helps a lot in keeping all of your timers lined up properly without any drifting. That's basically getting your opener Suiton, resetting early with Kassatsu, waiting out _two_ other Ninjutsu uses. This is, basically, a Ninjutsu on the pull, a Ninjutsu around 0:13, a Ninjutsu at 0:33, a Ninjutsu at 0:53, and then your TCJ around 0:55~0:57. This is super technical and goes against a point from above but this is one of those things that if you get in early then you'll build that consistency on your Ninjutsus and, specifically, your Trick Attacks without it going weird. The rest of Ninja is a bit of flexibility in that you're basically only worrying on two things with your GCDs: Keep up Shadow Fang without refreshing early and Keep up Huton without overcapping the duration. And, really, the Shadow Fang one is the only worry since Armor Crush can basically be thought of just making sure you use it 1 time _somewhere_ every 30 seconds. You can worry about the specifics of that (used at spots to make sure you're not Armor Crushing during Duality, during Trick Attack) later when you're not finding yourself Armor Crushing at 50s left or dropping Huton ever. Because of how this works out, this means your GCDs are basically something you plan around every 60s / 120s cycle so you get into very familiar patterns. Your oGCDs are important but due to how you want to use many of them, getting consistent with the above solves everything: Duality is just before an Aeolian Edge (don't worry about delaying this into Tricks, it's not worth costing you a use), Dream Within a Dream is every 60s inside of a Trick Attack, TCJ is every second Trick, etc.
*Assuming Bhava -> TCJ opener with 5th GCD Trick Attack. This is the bread and butter safe assumption but felt like giving the stipulation just in case!
A lot of that go over your head? Good! There's a lot to learn and internalize with rotations and there's absolutely nothing wrong with actually being overwhelmed by trying to learn them. It's surprisingly also just really different things that you work on getting into muscle memory since you just do them so, so, so much. Get the big pieces together. Practice. There's no harm in just actively trying to get better. Take it as fast or as slow as you're comfortable with. My general practice any time I want to pick up learning an opener is to know what my buttons are and go through the act of pushing them all at a slow speed. I learn a lot of what my rotations for multiple jobs are like with how specific parts of my rotation and a fight interact. Maybe that works for you maybe it doesn't and that's okay. For something I'm calling the basics I'm by no means calling them easy; they're just the foundation of everything. This is why I've always been a pretty big believer of just always "trying" since all content is some level of practicing your buttons and getting comfortable, and then you'll eventually reach the point of things like, "Oh, that's my fourth Demolish refresh so that's approaching Riddle of Fire being back." You'll get there eventually, just take it at your pace and use the tools available to you to get comfortable.