Marvel Heroes 2015. I miss my superhero Diablo so much, and the 2015 version of the game was the best version in my opinion.
I still see MH in my Steam games from time to time when going over older games to play. Had so much fun with that game, and I easily plopped down over 1000 in it until it closed.
But the big one for me, Auto Assault. God damn did I love that game, and miss it to this day. Still hoping beyond hope a private server or emulated server for it shows up eventually. That game was way ahead of it's time imo, though it did have major problems also I'll freely admit.