I voted for "Mind Flayer". Umber hulk would probably have been my second choice. Really, I don't have a great love for any of the "original" monsters. 75% or more of the critters on the list explicitly don't exist in my setting.
Beholder is definitely near the bottom of my list, along with piercers, owlbears, and ropers. They look more silly that scary, and really just aren't interesting at all. Original? You bet. Interesting? Not really.