Personally I’d revive Begone (Nplay.com). It was an FPS game built on Unity engine. I would to play it one more time just to feel the GUI, the servers and the community again. They had cool account systems and forums aswell.
Honourable mentions:
Minibash (a spin-off game to the game Toribash)
Fly like a bird 3 (gamevial.com)
And Attack on titan tribute game(Feng Lee)
Blacklight: Retribution
I had every single cosmetic and easily 3,000 hours in the game. It was my first serious competitive FPS other than Counter Strike: Source
Also, Hawken
Edit: Also, holy shit, Maple Story 2. So underrated that Nexon killed it off.