I think the all-around best movie that he did the effects for is The Golden Voyage of Sinbad. I'd easily say that the top tier is
The 7th Voyage of Sinbad
The Golden Voyage of Sinbad
Jason and the Argonauts
Clash of the Titans
If I had to pick a fifth one to round it out it would be Mysterious Island, which is a pretty good adventure movie with a Bernard Herman score, but the creatures in that one are just giant animals so none of them are among my favorite of his creations. Captain Nemo's plan to solve world hunger by creating giant crabs and bees could not make less sense. I should read the book one of these days.
I've never seen The Three Worlds of Gulliver
Which Are You Afraid of the Dark episodes have you watched? I probably remember them.