Mortal Kombat Armegeddon, call of duty ghosts Infinite Warfare and classicube.
jk i don't play classicube
Rather contradicting statement, Andrew.
Treasure Arena, for sure. That's a mighty good game. There's also Happy Wheels, it was popular, but the hype died down and now almost nobody plays it. IT does have a few popular levels everyday.
OH, AssaultCube is awesome. I love it. It's sad that almost nobody plays it in 2019. The Minecraft servers on there are okay, but I really like the capture the flag single player games. You should try Red Eclipse if you like AssaultCube.
ClassiCube, of course. I wish this game had many more players. I wish we could get some youtubers to cover it and get it big.
Halo: Custom Edition/Combat Evolved for PC. Still has players and even some people still make maps. Its a blast, haloce.org has some resources if you want to get into it.
Honorary Mention: Halo Trial. HT was the demo version of Halo Combat Evolved for the PC. It was limited to only bloodgulch for multiplayer, no custom gametypes, and the silent cartographer for campaign. People created modded versions that let you create custom gametypes, server features like the base game, and more. I hosted a server for a long time until gamespy shut down and the servers went with it. Tavrin, a dope guy in the community, created a fix and hosted the new server list but it required a patch and most people never found it. Its basically dead now with a handful of 24/7 empty servers and I miss it greatly. I created a copy of my old halo trial server and now am hosting it on halo custom edition.