These first two are pretty general. If you have a question about just about anything you more than likely find information from one these two sites.
WoW Head - Probably the most popular wow fansite. Huge collection of information. From drop rates to guides and even sharing transmogs. If there is something you want to know about wow I can almost guarantee it can be found here. But with that it can be hard find some these things like I only learned about the transmog thing recently.
Icy Veins - A very popular site for class guides. It's normally where I go but feel free to supliment it. They also have guides on dungeons and professions and stuff as well but it's not really what they're known for.
Next we'll cover addons.
Curse Forge - The most popular site for addons. 99% of addon can be found here more than likely. With the overwolf buy out you can use the overwolf program to automatically update addons from here.
WoW Interface - The second most popular addon site. Has most things curse forge has but has kind of fallen off compared to curse forge over the years. I do recommend the complication section though if you are looking for huge addon packs as those seem to have dissappeared from curse at some point. With their minion program you can automatically update addons from here.
Wago - Not so much an addon site but more of a supplement for other addons. If you have Weak Auras as addon this site is probably where you find your premade weak auras. But it also has stuff most popular addons including ElvUI, MDT, Total RP, and more.
Dungeon and raid guide sites/videos will be next. No real particular order on this one.
Mythic Trap - A relatively new site but I like it because how visual it is. Still a lot of stuff in the works but quickly becoming a popular option for dungeon and raid guides and the like(I believe MDT stuff is in the works).
Ready Check Pull - Another pretty new up and comer. Video guides for raids that I think are pretty well done. Sadly them coming out first with nathria means a lot changed compared to the fights now. They also do a podcast if you're interested.
Hazel Nutty Games - She does up to heroic guides for raids as well as dungeon guides. She also does wow news and stuff.
FatbossTV - Does raid video guides up to heroic.
Then lastly we'll drop off the odds and ends for like news and podcasts and stuff.
Taliesin & Evitel - A wow news youtube channel done buy a married couple.
Preach Gaming - WoW news and other wow related talks from Preach one of the older WoW youtubers.
Nobel87 - If you want to catch up on warcraft lore pretty much considered the go to source on youtube.
WoW Discord - You can also find links to other discords here. I don't feel like linking every single one but to sum it up there are class discords if you want help with you class. There is an achievement discord if you want help or friends for achievement hunting. There is a discord for trying to find those hard find secret pets and stuff. Basically there is a discord for every part of wow outside this general one but it can easily be found through this one.