Remember to say "please"... otherwise it is rude. ;) As a native speaker, my first reaction is "rude - ruder - rudest". A check in a couple of standard dictionaries agrees with that. It was interesting browsing the forums, as both forms seem acceptable. Some examples with "more rude" were perfectly fine. However, some of the commenters were non-native speakers so I really had to doubt some opinions. My suggestion is, try to use either form in writing and speaking, and ask for corrections from native speakers. Perhaps the usage depends on context. I seriously doubt it's a simple rule.

the most rude is correct