Wow! Tough call.
I voted Demon- Orcus, Shadow Demon, Vrock, Glabrezu, others. More demons come up in my campaign than the other two types. Demons, like spirits, tend to come up in OA-themed games quite a bit. They're flexible. I've even made up demons on the fly. They're a good all-purpose baddie.
However, I like the Bone Devil among others, although I've noticed in recent years there doesn't seem to be as much for devil type listings.
My favorite Yugoloth, formerly known as "Daemons" has got to be the Ultraloth. He was my favorite back in AD&D 1st Ed, and still runs high on my list of mid-high level nasties.
Good poll! Really made me think.