Every time I think I have an answer I think of another character. My top 5 are:
Inigo Montoya - Funny, honorable, hopeful...he's the heart and soul of the movie.
Prince Humperdink - He is just so loathsome and vile I can't help but love him.
Grandpa - You could put Peter Falk in just about anything and I'd love him.
Miracle Max - A small role but definitely a scene stealer.
and finally my number 1...
Fezzik - Andre was amazing in this role. He rhymed, he fought, he charmed, he did it all.
You have a great top 5 but two folks that you didnt mention Vizzini is also a hilarious scene stealer and often the first one I think about.
And Cary Elwes did a great job as Westley.