Absolute favorites go to Club Penguin and Pixie Hollow, back when the servers were still up! I still collect Club Penguin merch and I still love anything fairy :D
We used to play on Starfall and PrimaryGames back in elementary school, I loved them so much that I would play them at home too! I used to love Coffee Tycoon on the latter, even tho after looking it up recently its way more lacking than how I remember..
A little later, back in middle school, i would play a lot of flash games found on dA, mostly dress up and dating sims. There was this one user called Pacthesis whose work that I absolutely ADORED, mainly Number Days!
Then there are some i sadly cant remember the names of. I remember one game when I was a really young kid, I think it was either on cartoon network's site or I somehow found it through there, but there was this kid at home at night by their self, and you could click on things to get them to interact with it. I only remember an apple and something that caused an alien to show up or something. All I know is for some reason it absolutely mystified me lmao