Probably WAR.
WAR has the fewest hotbar slots of all the tanks, and you can fit all your "often pressed single target" buttons on one bar, "often pressed AOE" on half of a second, and your utility abilities/CDs on the other half of the AOE bar (with the short duration ones on your main single target bar for boss fights - when you need to weave a ton of mitigation is usually on wall to wall AOE pulls, so having it on your AOE bar makes sense anyway). Having two bars I can swap between with a single tap of Right Bumper (none of that "press and hold" shenanigans stuff) makes it super easy to fit everything on controller and have all your buttons where you need them. All of the other tanks go over 32 when you add up their abilities, role abilities, sprint, LB (which you do need in some rare 8 mans and when you need it [SoS, EW final] you don't have time to hunt in your menus for it), and potion (in raids/I swap this slot for mount in field)
As for the rotation, you get to learn it fairly early while leveling, as you have the complete "core" rotation by level 50. WAR uses a 1-2-4, 1-2-3, 1-2-3...refresh 4 system. Storm's Eye gives you a 30 sec damage boosting buff, but it can be stacked up to 60 sec. This makes it VERY forgiving if you accidentally refresh it early (like 5-15 seconds left will increase the buff to 30-45 as opposed to just 30 and the extra being wasted) and is super flexible. The 1-2-3 is static and easy to employ, and the third hit (Storm's Path) heals you, giving WAR consistent self-healing from level 26 while some other Tanks don't get theirs until much later. This can be useful when running dungeons at low levels or trying to solo older content that force syncs you, like Palace of the Dead, where some other tanks don't have their self-healing yet.
WAR's AOE is a straightforward 1-2 combo, with the 2 also giving you the Storm's Eye buff making it easy to keep uptime in AOE. Both single and AOE targets give you a 50 Beast Gauge spender (just use one for single target and the other for AOE, simple!), that also get boosted when you use Infuriate, WAR's "free 50 gauge" button, which is ALSO super flexible and morphs said spenders into more powerful attacks at high levels. You have one oGCD attack for single target and one AOE that share a CD, so you just use the one for the situation you're in. And WAR is pretty mobile, having 3 charges on its main gap closer, a second big attack gap closer when you use your big CD (Inner Release) at level 90, and has some amazing self-healing with Raw Intuition (level 56) and its upgrade, as well as Nascent Flash (which is the same thing for a party member but ALSO heals you. Raw Intuition heals you based on targets hit, meaning if you pop it in an AOE pack and start using your AOE combo, you basically can't be killed because you're healing for your entire health bar every time you use either of your AOE attacks.
And WAR's invuln is the most utility worthy of the bunch, since it has the shortest CD. It isn't a true invuln like PLD or GNB (where it prevents taking damage), but it prevents anything killing you (prevents falling below 1 HP), and the short CD means it's the only one that can be used up to 3 times in hard content (the other tanks only get 2 uses out of theirs). It also doesn't have the nasty drawback of DEATH like DRK's does if you don't heal up...and WAR can super fast heal up anyway between Equilibrium, Raw Intuition, and Storm's Path.
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Does that sound like a lot? Well, every other Tank I could write 3x that much.WAR has a simple, straightforward, and flexible single target rotation, even easier AOE rotation, the most massive self-healing/sustain in the game, some party healing utility, and equal or better slate of defensive CDs to all the other tanks.
Hands down, I think it's objectively the easiest tank to learn. It may not be flashy (okay, Fell Cleave spam and Inner Release/Primal Rend ARE pretty flashy...), but it has all the tools you need, the tools are easy and make sense, and it gets the job done.
Some of the others like PLD can start easy but get complex - especially when trying to optimize with that cursed opener - or really busy like GNB and DRK (in its opener/burst window). WAR, on the other hand, is pretty consistent. It gets flashier and stuff, but most of the time as you level, its abilities upgrade RATHER THAN have additional things/buttons/abilities tacked on making a more complex rotation. This allows it to remain pretty simple all the way to and including at level 90. It's easy to learn and pick up, and pretty easy to get good at, but there's still a little nuance if you want to push things to perfection and grow into it, and its utility and invuln power make it a great choice. It's not an "easy to learn, but no one wants it at level cap/Savage raiding" Job - it's easy to learn AND useful in high end content.