Recommended gamemodes for two player only game?

[Ran_TH]

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

[RobyYe]

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.

[ash47]

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

[343N]

Onslaught: Evolved
build base together
kill npcs together
die together
<3