Some theorems are magical: their hypotheses are easy to meet, and when invoked (as lemmas) in the midst of an otherwise routine proof, they deliver the desired conclusion more or less straightaway—like pulling a rabbit from a hat.
Perhaps to meet the sheer linguistic poetic requirement, Well-ordering theorem which states that "any set can be well-ordered" remains 'hands-down' as any meta-logician's top answer. In terms of historical perspective it has taken the logical house by maelstrom (with seemingly counter-intuitive offshoot of Banach-Tarski paradox at first) resulting from Axiom of Choice(AC) as mentioned by John bell in the SEP chronology:
1904/1908 Zermelo introduces axioms of set theory, explicitly formulates AC and uses it to prove the well-ordering theorem, thereby raising a storm of controversy.
