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.
I agree, an all-around great cast.
Personally I think I like Westley best. He is able to play off of every other character and hold his own. I particularly like the bit about "if only we had a wheelbarrow" and Fezzik says what about the Albino's wheelbarrow and he says "why was that not listed among our assets?" I don't know why that just kills me. Also "there's a shortage of perfect breasts in the world, it would be a shame to damage yours" and the "to the pain" speech. Who else could play Westley?