Beholder for me. I think one important aspect to this is how dangerous and memorable an encounter with a monster is. A Beholder has multiple dangerous attacks and is from a wholly different reality that is innately dangerous to the main reality just by existing, making Beholders an existential threat. I also think they look very alien, gross, and intimidating (at least in the better art). Finally, they are very unique to D&D itself. The only thing even remotely similar is the Cacodemons from Doom, and that's only slightly similar. Kobolds, Owlbears, Rust Monsters, and mimics are a little too small potatoes to really make an impression. They're all great, just don't have to extra oomph that some others have. Dragons also fit the bill but are a little too generic to feel quintessentially D&D.