There's no way to tell. If you ask mainstream media, they would say SN. There's also the fact that there are lots of non-Americans here or otherwise people who don't have access to guns; if this were a US centric forum, I bet guns would be at the top for sure.
I think hanging and SN are up there. Most other methods of poison are kinda unreliable. Firearms for US-based bus passengers, for sure.