Personally I’ve played a hell of a lot of HexArena. Just a simple, low intensity game that runs well and is maintained :))
And before anyone says it, chess and go are great browser games!
For me this is a hard toss up between Pizza King too and the Submachine series, the Submachine series is an amazing escape/puzzle style series that has a really good story
and Pizza King 2 is the best management style Pizzeria Simulator game that I have found to date, many of those management style games focus more on the cooking aspect than the management aspect but this one takes the cake with being an actual management Sim instead of a cooking
I’ve played alot of paperclips in classes, it just looks like a random website but it’s a really fun idle clicker thingy it’s at https://www.decisionproblem.com/paperclips/index2.html
I replay candybox/candybox 2 at least once a year. Fantastic incremental ascii games
Amazed no one has mentioned Antimatter Dimensions. It’s now on Steam, but was originally on a few sites (I played it on Kongregate). Fantastic, and now with the latest update and the ‘aero’ skin and hidden games, it’s multiple games (Including the original Paperclips) inside one :D