I think the popular Elvui is the best UI addon and I couldn’t live without it… completely replaces the standard WoW UI. Icy veins pro players all use it. Check you tube/google if you wanna to see how it looks. The default is so much better than the standard WoW UI. If you have Elvui, you won’t need bartender or other supplemental UI addons.
It’s easy to customize with so many options. To name a few, drag anything anywhere, resize, add new action bars with various size and shapes, automatically sell greys to npc’s, change nameplates, unit frames, Fonts, chat windows, colors, increase the size of your mini map, etc.
Google “ELVUI download” and it’s in the first link called Elvui - Tukui. I can’t post the link here. Elvui puts its own button on your WoW main menu.
The consensus is the twitch app is the best way to download and update addons, but I’ve never tried it…will try later. If you want to do it manually without the app simply download the addon, copy the folders, and paste them to your WoW/retail/interface/addons folder. Takes like 20 seconds
I would also get the Details addon from curseforge, which is the most popular dps/hps addon. Weak Auras 2 for buffs, debuffs, dots, resource bars, cool downs, etc. lastly, “World quest lists” is a must have at level 120. It shows lots of extra world quest info and also gives you a clickable list attached to the map. I get all addons from curseforge with the exception of elvui.
For rares, two of them are Rarescanner and Tom Cat Tours, but Tom Cat may only be for 120 can’t remember.