Just a note, I'm still in the mist of progging TEA, so I might change this list later on.
Ultimates are also the hardest fights in FFXIV, I thought I'd rank them depending my experience tanking each of them. 3. UWU (The Weapon's Refrain)
Most of the mechanics are straight from extremes and so they'll be familiar with many people, and they are repeated A LOT.
Provided you've read a guide, it's the easiest ultimate (as the ultimate was designed to be run blind). But reading guide will drastically cut the hours to prog UWU.
DPS checks are very lenient in this ultimate, many of the mechanics are often done moving as a group.
Plus it's the easiest ultimate to reclear consistently, you can often get away with a couple deaths here and there, provided your DPS is high enough, you will still clear. 2. TEA (The Epic of Alexander)
I'm still progging this ultimate but so far the first two phases are quite difficult and so is wormhole mechanic.
Some mechanics require a lot of memorising where to stand, for example limit cut where each players get a number 1-8 and they stand are different parts of the arena to bait cruise chaser without clipping others or being clipped by the explosions.
But after the 2 phases, it does become a lot easier or at least the mechanics are a lot easier to solve.
The DPS check certainly is a little tighter than UWU, but we seem to be skipping mechanics regardless (most likely due to relic weapons and dungeon gear having superior stats nowadays).
I'm not too familiar with Alexander mechanics, however we do seem to be making quicker progress then UCOB...1. UCOB (The Unending Coils of Bahamut)
The most difficult fight in FFXIV.
The nael phase in this fight is the most mechanical intense fight ever, there's so many debuffs going off, nael is shouting these "quote mechanics" and tankbusters are going off... it can get a little hectic...
Furthermore, every run of UCOB is never the same due to the randomness of the mechanics.
For example, in tenstrike 4 random players get earthshakers and they should not overlap or they die. Then the remaining 4 players get earthshakers immediately after, communication is needed.
Bosses also autoattack A LOT, and they hurt A LOT.
Furthermore, the Golden Phase is quite punishing, if people die they lose the phoneix debuff (which allows people to do double damage) which makes the DPS check all the harder to beat.
UCOB was the first ultimate in the game, and it still remains the hardest one in my opinion.