The progression is trace<info<warn<debug<err<fatal<always
Usually, the most useful one is level = * err or level = * error because it will show you the errors you make (like spelling mistakes and so on) without giving a lot of unnecessary info. However, if you're creating triggers or scripts and want to test them you should use level = game.script.* trace as it will yield all the info about the scripting side of the game, including triggers.
I don't think there are any debug messages in the retail exe so debug is the same level as err
The warn level can be useful sometimes if you want info about missing files and so on but you'll have to dig through a lot of stuff to find it.