---the most wanted class depends on role, and most categories have a few competitors.
In raid, the composition of the group is first and foremost geared around acquiring the buffs/debuff as to fully maximize everyone's output. This constitutes the majority of slots taken and leaves 4~6 slots open for choice fills. This often results in the best output classes available across the players you have getting doubled up on/brought to fill those slots.
--In m+, even small variances in output or just having a sufficiently broad utility kit along with high damage often yields certain combinations of setups.
There is variance and room to account for class preference and skill, and these aren't the only viable classes. You can run X class, you just need to have the group play around it and there will be a point that it falls off but as the logs show that theoretical upper limit is in the high 20s. Just because the tanks being used in +30 keys are DH/Paladin doesn't mean those are the only tanks that work, for example.
--One of the Race to world first raid leads talks about why the raid comp looks the way it does in a video I'll link here, its interesting to hear the thought process in depth.
https://www.youtube.com/watch?v=INjo6oAQ2kc
If you want to see how many of which classes are being brought to keys, i would recommend raider.io, it tracks keys done and is fairly accurate for the highest of key levels, though once you get too low blizzard stops hashing data and they cant track everybody.
--The first is a link to the subsection of their website that shows highest keys done and class populations
https://raider.io/mythic-plus-rankings/season-df-3/all/world/leaderboards
--This second link is with it specifically tuned to 25+ keys only
For the very freshly returned or still learning, consider checking out https://www.wowhead.com/ for general starter guides, and your class discord (some are more useful than others). take wowhead with a grain of salt, its usually written by some of the best players but might not have all the inner tips and tricks but is often a great place for start info.
A more advanced tool possibly for later, https://www.warcraftlogs.com/ this leads to several others as well, discussed below. This is the direct data for runs to parse through (often referred to as "parses") where you can see what ppl did in raids.
--It is one of many tools to analyze data, other such ones being:
https://www.wipefest.gg/?gameVersion=warcraft-live - Wipefest might give you easily readable ideas of how you did on mechanics.
https://wowanalyzer.com/ - This uses warcraft logs, this might be accurate or useful for your class, uses a parse to tell you what you might be able to improve on. not always 100% accurate.
https://lorrgs.io/ - Another warcraft logs metadata tool, you can see what the highest people use and when they use it to get an idea of what be do
https://bloodmallet.com/ - Another useful tool for reference data
There are many others, such as https://murlok.io/ and all are useful tools to get data from and many might be specfiic to class, such as some great basic info for Holy Paladin (from one of the best M+ H.Pal runners around) is https://wingsisup.com/df-talents.
--We all know blizzard's UI isn't the best or doesn't always address some elements we like in the best way, and so over a decade ago we started augmenting the game via addons, macros and the like. Popular UI over halls let you completely change the look of the game should so you wish, such as ELV-UI or similar, or just a specific component like your action bars.
There are various ways to update your addons, such as manually or some allow for automatic self updates, but for those that don't there are several addon update packages available. CurseForge is popular, I have also seen WowUp, and the WeakAuras have their own companion apps to keep your auras updated (as if a WA gets outdated / game elements it interacts with change, it'll spit errors till you turn it off)
There are many others, but those are the ones i have the most experience with using, and such can give advice related to their usage, feel free to link other alternatives in the tread.
There may be specific case scenarios, like smolderon, where turning particle density down will help. also, some instance graphical scenarios are helped by using color blind mode, as it forces similarly colored effects into more different ends of the spectrum, or inky black potions turning the general lighting down but then certain effects glow more.
--Of potential additional assistance is this video going over the options and some variance settings. some minor differences from the github post, but verbally gone over for those whom prefer information presented as such. https://www.youtube.com/watch?v=-vR6nJLLoe4
-Zaughlin