A Flash game I used to play a while ago was this deathmatch game all about Robotnik and others in Robotnik machines fighting to the death through transforming into other boss machines from stuff like Sonic 1, 2, 3, CD, etc. I used to love that game and it even had multiplayer!
But apart from that, I also used to play Suepr Smash Flash 1 and 2 all the time, but everyone else already knows those games lol