[llllllllllll]

It's good practice to check your intuition on things like this against a searchable corpus. According to http://corpus.byu.edu/coca/, "most wicked" just barely edges out "wickedest" in terms of frequency, 22 to 20.

An example that strongly goes against the rule:

simpler (3185) / more simple (153)

simplest (2191) / most simple (87)

[jameshart]

It's a practice, but as an English speaker I have the privilege of making my own corpus up as I go along. When I say I find I prefer 'most wicked' to 'wickedest', I'm expressing the subjective opinion that I think I would fall in the 22 part of that corpus, not the 20 - if anything I wrote about relative wickedness were to make it into the COCA corpus that number would go up to 23.

But that would be true even if I were in the minority. The corpus is just going to reflect usage, and my usage is as valid as any other, so... while it provides some sort of peer validation, it doesn't answer the question of whether one form or another is 'more correct', only whether one is 'more common'. Or commoner, maybe. No, I'm going to stick with more common.

[camillomiller]

As a non-native English speaker, I find this to be a precious suggestion. I'm going to try and use it for those situations of uncertainty between two forms. I usually resort to some specific google search, but that's often ineffective, especially if the "wrong version" is pretty common among a subset of international speakers.

[baddox]

"Simple," "narrow," "clever," and "quiet" are usually listed as exceptions to the rule, and their suffixed superlative forms also usually appear in dictionaries.

[sgustard]

And a single-syllable counterexample: funner (24) / more fun (1860)

[baddox]

That analysis is probably confounded by the fact that "more fun" can be used with "fun" as a noun, as in "We had more fun after you arrived."