Number Five from Short Circuit. I was excited when this movie came out, as it was the first video I had ever seen with a robot that actually looked like a robot, not a human in a suit or a mobile trash can. Decades later I suppose few people will really grasp how ground breaking that was. The fact that the operators managed to make him a very expressive character was just bonus.
Smarf.
Think you meant Snarf there, Andy (IIRC…you may be right)
As for me, three that come to mind are:
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Grig, the alien sidekick in “The Last Starfighter”
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The beast from “The Beast from 20,000 Fathoms”
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And the giant Ants from “Them!”
I would have thought you’d put in a vote for your cousins the Metal Munching Moon Mice from Rocky&Billwinkle.
Well, if we’re including characters, like Grig or Gollum, played by humans in costume or with added special effects, then I’m going to nominate Spock and Chewbacca. 
Nope, I definitely meant Smarf, though he was in a short video and not a movie.
Chinchilla, the Wookie Monster
Artie Deco
4-Q-2
Ah, my mistake, I thought you were referring to the character on the cartoon “Thundercats”.
Serkis should have won the Oscar for Best Supporting Actor for his performance in The Two Towers, particularly the dual-handed soliloquy, but the rules didn’t allow it.
I’ve been in love with Kermit the frog forever. I’d marry him if I could.
Better not let Miss Piggy hear you say that. 
Kosh-A darned puppet and almost cried when he died. (Sorry about that to any who didn’t know.)
The Great Gonzo!