Several people suggested this topic, and there are a LOT of contenders. I’m just gonna list 5, in episode order:

• The title character in episode 107- ROBOT MONSTER. You can’t ignore an iconic bad monster.
• The the eye creatures in episode 418- ATTACK OF THE THE EYE CREATURES. If they attack you, just knock the heads off, since they seem to be draped casually on their shoulders.
• The title character in episode 606- THE CREEPING TERROR. The music is scary, I admit, but this is another one of those monsters where the victim pretty much has to actively climb into the monster’s mouth.
• Ortega in episode 812- THE INCREDIBLY STRANGE CREATURES WHO STOPPED LIVING AND BECAME MIXED-UP ZOMBIES. Actually seems like a pretty cool guy.
• The title characters in episode 907- HOBGOBLINS. Any monster you can drop kick into the next county doesn’t seem that fearsome.

Honorable mention: MITCHELL. The love scene was monstrous.

Oh and I didn’t include the monster in epiosode 421- MONSTER A-GO-GO because there WAS no monster.