Personally, my favorites are Robin Mask and Warsman.
Coming up as close seconds are Ashuraman and Sunshine.
Buffaloman would probably be third.
Kinnikuman himself, and Neptune King are actually a ways down the list. Fifth or sixth place at the best.
I'm currently reading the Manga for the first time, and the character who surprised me the most is Terryman. I was not expecting to like him as much as I do.