Worst (e.g. least convincing, threatening or scary) movie monster ever:

• Ro-Man
• Reptilicus
• The Catfish-man from Blood Waters of Doctor Z
• The Green Slime
• The Turkey-Headed Vampire from Blood Freak
• Tabanga (From Hell It Came)
• Tor Johnson in anything
• Phantom of Krankor
• Cowboy Neal

(there are others - maybe this is too obscure?)