I'm just going to go through the addons I use and why I use them. I'll preface this though with the customization options of WoW UI are it's greatest strength. You'll see it even at the highest level where people will have extremely different UIs so don't be afraid to mess around find what works.
The first thing you'll want is a great addon manager. Most addon can be gotten from curseforge which means theirs is probably the most popular. However I use addons tha aren't on curseforge(plus I don't like overwolf) so I don't use it. You can find a list of community made alternatives HERE. I personally use CurseBreaker for its simplicity but WoWUP and Ajour are probably more popular because they look pretty.
Alright I have a lot of addons (44 according to cursebreaker) but many are actually addons adding on to my addons. You could easily have more or less depending on what your goals are. Like I don't really care about PvP so don't have PvP dedicated addons. I'll try to start with the bigger or what I consider more important ones and move my way down.
ElvUI - Is probably one everyone has heard of. Many avoid it because it does a ton. This comes with many benefits but also feels very overwhelming for many. I tried to avoid it myself but kept ending up needing more addons to do things I felt I wanted that well elvui can do just by customizing stuff. Now I supplement ElvUI with a few smaller addons which I'll quickly cover here. NoobTacoUI as I like it more as color and layout starting point. But it's basically just a collection elvui settings and not required if you're willing to set up ElvUI more yourself. Windtools does a lot of funcitonal things. It does those little announcements you see in raid and dungeons with me. It adds some bars that automatically add items and stuff so you don't need to open your bags. It does a lot and I don't use it all but highly recommend it. That's about it. I use AddonSkins and ElvUI Adibags but that's for better integration with other addons.
BigWigs and LittleWigs - Now DBM a is more popular boss mods option but well I just prefer BigWigs. By default I find it much less annoying and more stable. I also find it way easier to customize the fights to my needs. Like counsel is a great example. I have cast that we need to interrupt make wolf howl sound and the attendent make an air horn sound which I don't think are default. Basically if you feel you want more in your face like DBM I suggest looking into the settings of BigWigs or DBM(as it's also extremely customisable but I didn't find it as user friendly) instead of using both. There is also packs for old content but I'm not going to link them all despite having them all.
GTFO - Is a simple addon. It yells at you when you're standing in bad stuff. If you find you're often standing in bad stuff and not noticing get this this addon.
Weak Auras - By it self this addon doesn't really do anything. It's a tool to add things to your ui without needing to learn to make addon basically. You can find premade stuff on Wago (which also includes stuff for elvui and MDT which have or will be talked about).
From here we're going to get more into perfsonal preference/quality of life addons. They have more than likely have many alternatives or are very not needed IMO.
AdiBags - A very nice bag addon imo. I've tried many others but this one just works for me in the way I want with very few setting changes. If you use elvui I suggest getting the elvui version I linked above so it integrates better but not required. I won't link them all but I will list the addons I use for this addon to make it nicer for me. I have "Battle Pet Items", "By Expansion"(but disable the shadowlands portion of this one), "Fishing items", "Ren's filters: reputation", "Shadowlands"(Why disable the shadowlands portion of by expansion), "Zone Items", "archaeology", "Heathstones", and "Lootboxes".
Pawn - Tries to make easier to tell if something is upgarde for you based on stats instead of just ilvl. IMO as the devs streamline gear more with each expansion its usefullness is lessoning as we go on.
AskMrRobot - Is my sim tool of choice. Simply because I find it easier/more userfriendly. It needs to be partnered with their website to be useful. They'll also generate pawn codes for you. Popular alternative is Simcraft with raidbots. These tools are about finding those minor gains you aren't sure about. Have something with the same ilvl but different stats and this will basically tell if you played perfectly which is better. If we get enough people using either of these I believe both offer guild/group discounts for their paid options.
Decursive - Is a great tool for classes who want to easily know when they can remove a debuff off allies. I used it on my mage but well it's useless on my rogue. Eitherway if you want to make removing stuff easy I highly recommend it.
Clique - Now this one I don't actually use personally. It makes it easy to make stuff target what you're mousing over instead of what you're targeting. I basically think it's a requirement for healers though. But since I just play healer these days i don't have it installed. Now there are altenatives such mouseover macro's or I believe vudu and others support its functionality by default as well but i just liked it for its simplicity.
Can I mog it? - Makes it easy to see if you need to equip something to gain its transmog.
BugSack and BugGrabber - When you get to enough addons you're probably goign to get some errors that most likely can just be ignored. So we have these to keep track of them or simply keep them out of our face.
Guild Lottery - Addon I was using for the guild lottery. Let me know if you want me to start these back up or if you have a format suggestion for them.
Guild Bank Search - Guild banks can often get messy so this makes it easier to find things.
Identity 2 - You should all get this so who I know who I'm talking to in guild chat!?!!? This is what adds [Alex] to the start of my guild chat messages automatically.
Misdirect Helper - Only usefull for classes like rogue or hunter that make their threat generation target someone else but makes that easier basically.
Molinari - Makes milling, prospecting, opening locking boxes, etc easier.
MrFish - Makes starting, stopping, and fishing much less work.
PersoLootRoll - Makes rolling and giving away stuff in dungeons/raids super easy.
Raider.io - Helps you see what raids and mythics have finished.
Ravenous Mounts - Getting the same mount all the time is boring.
Skada Damage Meter - Details simply tries to do to much for my prefrences.
Venture Plan - Makes it easier to tell if a SL mission table mission will succeed or fail.
WoW Instant Messenger - Makes keeping track of Whispers easier IMO.
Zygor Guides - This addon does cost money. I don't really recommend it. But I do have it because it does a lot of things that I would use other addons for all in one. I'll list the alternatives that I would recommend for most people below.
World Quest Tracker - Makes seening what world quests are available and their rewards.
Azeroth Auto Pilot - Makes leveling through questing super easy.
The Undermine Journal - Tells you what stuff is selling for on the AH.
All the Things - If you want to collect everything well this helps tell you what you're missing.
And that's about what's included in Zygor in different addons.
Hekili - Helps you optimize your damage rotation.
Leatrix Plus - If you don't use elvui or want to keep the standard ui I highly recommend this addon. It does a lot of small things (faster loot, auto repair, and more) that I consider essential QoL changes.
Mythic Dungeon Tools - Useful if you plan on tanking Mythics for route planning. Other specs probably don't need to worry as they shouldn't be pulling...
And that's where I'm going to stop for now. I'm sure there are some I could add but I already have a lot frivoulus stuff IMO. Again basically make your UI work for you . I have a few weak aura's but those are the kind of things you just say hey I want to know when x is happening or something you look for it. Like I have one for when lust and battle rez's are available. So if you feel there is something the WoW UI could be doing better there is a great chance someone agrees with you and already made and addon or weak aura to make it better. So experiment and find what works for you.
Here's a screenshot of what my UI kind of looks like during a boss fight. The buff/debuffs are off especially on raid frames but I couldn't be assed to figure out how to modify if or the preview option. But elvui is extremely customisable and I have a lot stuff set to my preferences.