I just wish the Mission Generator was available for single-player use, too. It's a decent, simple tool, but it's tedious to play it alone on x1 Time Compression for no reason.
It depends on the genre, really.
FPS: MP all the way. Stupid or nonexistent bots, smart humans (for the most part
)
RTS: Scenario Design and scenarios.
Sim: SP all the way.