It's hard to make best-of lists because it's an activity that's always sure to attract arguments. But I'm assuming that not many would disagree when I say that these three websites are the most important for me. Not a day goes by without me accessing at least one of these essentials.
Being tagged as my search engine of choice on my browser, I don't even need to go to Google's homepage to use it. I just type my search terms on the address bar and Google results automatically appear. There are other search engines, of course, but I, nor anybody I know, use any of them.
All of the popular websites have now morphed into apps, and YouTube has made quite a smooth transition. Even the rise of the streaming industry can't stop the ubiquitous popularity of the pioneer video sharing company, even as their paid services lag behind more popular streaming services. You can be sure their ad revenue more than makes up for it.
Wikipedia has gotten a lot of bad rep as an unreliable source of information for students and academics who do research. Personally, I believe the accusations have been greatly exaggerated. Yes, there may have been some tainted entries-- it's part of the risk of being crowd-sourced, but I venture that 98% of its content are accurate, which include the stuff that I've searched on it so far.