CORRECT Grammar

B and D are all acceptable in their special conditions of usage.
C would be an answer to a question (as an example) but only in a few
instances

Man, when "...an good judge...." okay

Never!

Sorta like "I be'

The correct answer is "B" . . . I am not a good judge of poetry.

"Some" slightly neg implication, like "I'm not some common thief"

My BH says "the" okay, especially if there were other specific instances of judging to which I'm comparing myself

Right, it all depends on context. Say, for example, you were at a poetry reading exhibition where the winner (by declaration of the judge) would get a prize. After a given recital someone, apparently mistaking you for the judge, comes up and asks: "Is that one going to win the prize?" Then "the" would be appropriate. I might be the judge for the upcoming prose presentations, but not of the poetry sessions, ya know?

The only answer that would never be right is "an." Given the most common and frequent circumstances "a" would be most typical. But "a" is not THE right answer for all contexts.

"Some" slightly neg implication, like "I'm not some common thief"

Often, Dale, but not necessarily always. Using "some" tends to indicate that there is such a class people out there, but you're not a member of that class.

If I was asked a question that was too abstract or complex for me to answer, I might say: "I'm not some rocket scientist." Such a characterization would be self-deprecating but there would be a positive connotation about the class of "rocket scientists."