I do think it's important to have a minimalist UI and to reduce the amount of things on screen. But every player has to decide what they need to see, that's why the default shows everything.

This is my UI: https://i.imgur.com/tbgA92L.png

During combat I focus on a small area surrounding my character (the middle of the bottom half of the screen). Enemy list and party window are to the left and right so that changes there still register in my peripheral vision. Focus target is always set to the boss and put near the party window, so that when I look at the party window I am still aware of what the boss is casting. I have attacks on the right of the minimap (bound to MMO mouse) and defenses on the left. Defenses are on the left because they are not pressed as frequently as attacks, so they do not overburden the left hand which already handles movement.

I hid my parameter bar and instead look at the party window. My HP is displayed over my head regardless, so even without looking at the party window I know if I am in danger.

IMO the following are the highest priority and must be clearly visible:

• Bottom of your character's feet (to position accurately)
• Boss's cast bar ("progress bar") - IMHO this is absolutely critical and you must know at all times what the boss is casting. Recommend to separate it from the target info display, put at 200% and in the middle of your screen
• Your own buffs and debuffs - since many bosses have mechanics handled differently on what buff you get (eg Tsunami 1 and 2, WL1 & 2)
• Target's buffs and debuffs - same reason as above. eg Ramuh's Stepped Leader and Tribunal Summons behave differently depending on whether or not he has the Fury buff.
• Cooldowns and ogcd's
• Party list - to tell if anyone is in trouble
• Enemy list - to check for the presence of adds. Many boss mechanics are also cast through untargetable adds (E2S hand of erebus, E4S bomb boulders, E8S mirrors) and the cast bar shows up in the adds list, allowing for more precise timing
• Chat window - in case party is talking to you

IMO the following can potentially be hidden:

• Inventory grid - you can always open your inventory.
• Exp bar - you can look at your character sheet instead.
• Parameter bar - you can look at your party list instead. Your own HP can be displayed even if the parameter bar is hidden (Character config > Display name settings > Own > HP bar settings)
• Duty list - should be set to "Hide duty list during instanced duty". But you don't want to hide it through the HUD editor, since it displays time left in the instance.
• Main Scenario Guide - at the minimum, should be set to "hide when MSQ complete".