Ah, Flash games, There's so many I enjoy, I'll try to keep my list down to a minimum, but it will be difficult.
One of them is a game called Sonny, where you actually play as a Zombie and fight to survive in a world where everything wants to kill you again. It's a short, but very thrilling Turn based RPG.
I really loved Rebuild a 4x Turn based/Real Time Strategy hybrid about survivors of the Zombie Apocalypse, and was excited when the developer decided to make an "official" version on Steam/GOG in the form of Rebuild 3: Gangs of Deadsville which I ended up buying.
Bullet Heaven was also a fun one which is a Bullet Hell-type game. I actually played it before getting into the Touhou series which Bullet Heaven naturally got inspiration from.
Doodle God was good since you could combine so many different combinations of elements to make yourself feel like a scientist.
Burrito Bison since it feels satisfying to smash up candy and gummi bears (Because I'm not a huge fan of Candy overall unlike most people).
K.O.L.M. because it is Kind of Like Metroid.
Pandemic 2 since it is a disease simulation where you create a disease and evolve it to wipe out humanity. It also seemed to serve as inspiration for what became Plague Inc.
And then Phage Wars 2 I've spent way more time on this game than I probably should, but it's so addicting due to it having a similar nature to 4x games where you conquer other Cells with your virus and destroy other viruses.