Mummies and Werewolves are both easily recognized, and classic. The former would be easier, but the latter would be cooler.
A Frankenstein's monster-esque flesh golem could work if it was done right, but might be confused with zombie by the uninitiated.
Mind Flayer would work despite not being recognized, but even without cephalophagy, you'd lose your job if you tracked brackish water through the office and left the lingering scent of cinnamon and onions everwhere you went. Without the special effects, it would lose effectiveness.
The same is true for most of the maggot-ridden undead - too gross to be allowed.
The "acting costumes" don't win contests, but it still might be worth trying the witch doctor, with staff and fetishes and skulls and beads, etc. - just open you eyes way too wide and use alternately fluid and jerky movements while muttering under your breath.
Ultaloths require too much background. Canoloths and nycoloths require too much fx. Arcanoloths are just werewolves in finery.
Lich is just a zombie sorcerer and will only win "best individual lifetime achievement but now washed up and out of ideas"
Nazgul is almost timely, but not.
Myconid will only get "What are supposed to be - toadstool from the Mario franchise?"
Gelantinous cube is too hard to pull off.
I think you should go with mummy. Make sure that you include smells - cinnamon and naptha and so forth.