Stumper

Noun

 * 1) One who stumps, or speaks, or orates, as a politician.
 * He's not yet well known, but he's a good stumper.
 * 1) A difficult puzzle or problem.
 * Global warming is a real stumper.

Thesaurus
baffling problem, brain twister, campaigner, crossword puzzle, crux, electioneer, enigma, enigmatic question, floorer, jigsaw puzzle, knot, knotty point, mind-boggler, mystery, nut to crack, perplexed question, perplexity, poser, problem, puzzle, puzzlement, puzzler, question, question mark, sixty-four dollar question, sticker, stump orator, stump speaker, tough proposition, vexed question, whistle-stopper, why