I'd have to say Google. I've been using it for literal years. Not to mention their other services like email and the maps.
But besides that, I visit places like GameFAQs semi-regularly for when I need help with games, and I use Bulbapedia and Serebii on a semi-regular basis too when I need information about Pokemon stuff. I've used them all off and on for years, and they're usually my go tos when I need help with something.
Sonic Retro is the website I've been visiting the longest that's still around. There were a few others I would visit quite frequently but they've shut down.
The website I’ve been visiting the longest is probably YouTube. It’s been a go-to for everything from learning new skills to entertainment for well over a decade.
I have been visiting the Internet forums more nowadays and I can say that they serve as my second home. I also use other platforms like YouTube and Google. They are so important too.