Regular and Aspire mapping contests for the win.
As I can sing (ish) Osu!idol would kinda be okay too, but it's not really the thing for Osu! in my opinion, and the same with art contests (even though my art is horrible), but again, not exactly the thing for Osu!
I think we should have a lot of mapping contests, since it can benefit the community a lot, with making more people want to make their own beatmaps etc.