Recommended gamemodes for two player only game?

As the title said, can anyone recommend a fun gamemode for two players?
Prop Hunt seems fun, but getting bored at some minute.

Sandbox. I’m serious here, it’s fun if you have someone with you to do random shit. Building alone is already somewhat fun, But lonely.

Although not Garry’s Mod related, the Portal 2 coop gamemode is awesome and two players :stuck_out_tongue:

Onslaught: Evolved
build base together
kill npcs together
die together