Artix Entertainment, a THOUSAND TIMES, Artix Entertainment.
These are without a doubt the highest quality flash games that I have ever seen, and I am having trouble of thinking that there are some that could be higher.
Their games tend to be fantasy games that parody other works. They are flash, 2D, and are all completely free to play, but you can pay to recieve special privilages such as more powerful items and quests.
Adventurequest gets either a weekly or a monthly update, and has been going for 11 YEARS. Overwhelmingly large game. Fantasy based, kind of 'pick your own aventure'. Not the best of their games in my opinion.
Dragonfable has been going for half that time, but gets an update every Friday. Fantastic game, very funny story, easy to get into and hard to get out. Weekly updates for I think 7 years, large game.
Mechquest is mech-based, you customize your mech, and fight other mechs. Not bad, but not great. 5 years of weekly updates, I think.
Their newest game, Adventurequest Worlds is a MMO. I have sunk more time into this then Minecraft, Terraria, and Fallout 3 combined. This game is ADDICTIVE. The game is much more pay oriented then their other games. Members get countless perks, items, and areas. Is 4 years old this October, and has gotten an update every Friday of it.
I played Dragonfable for about 4 years, and Adventurequest Worlds for 3 before I quit because I was sinking too much time into them. I played these games more then my pc or console games.
Try DF or AQW at least, they are the best free games I have ever played.