I'm gonna have to agree about 5cp. It's my favorite mode too. The back and forth is best in 5cp.
But I'm gonna have to go with foundry for my favorite map. Not because it's well balanced or anything, I just like it. I think it's neat. I like to try to push people into the cauldron, and have long games (but not always games as long as powerhouse provides.)
Also runner up and a bit of a hot take: pass time is really fun. It's too bad we'll never get an update to it because of copyright issues, but you can get into some fun back and forth, and it's a mode that is severely underrated. District is my favorite map from that mode.