This site has a few high quality browser games. The one I come back to is X Type, a bullet hell shoot-em up that has ever expanding enemy ship sizes, and never ends. It gets hard fast.
I also like Xibalba, which is a Doom/Wolfenstein style game playable in the browser.
A simple but elegant io game. You are a ball, and you want to knock other balls to the ground.
One thing I like is that rounds in small, 4 person lobbies, rather than the massive worlds of other io games. Although you can’t really make friends, you can know personas, and it’s more personable.
I like to link it without the ending title, like store.steampowered.com/app/1944240/ because it’s funnier when people can’t see the game title in the link.
Absolutely obligatory, the simply named “The Game” is a work of art, and truly a life changing experience. You’ll never think about things the same after experiencing “The Game”.
A fnaf fangame that is close enough to feel like fnaf, but has a twist: Every single level also involves a puzzle. While trying to survive enemies fnaf style. Although I’ve never played this game, I LOVE watching it on Twitch. I like to call it “Human’s can’t multitask: The Game”.
Also by double speak games, and open source gridland is a variant on the match 3 style. During the day phase, you accrue and store resources, and build stuff. During the night phase, you fight.