How many letters does the top answer for “incapable of being overcome, challenged or refuted” have?

The most common answer is impregnable (11 letters).

Crossword clue: incapable of being overcome, challenged or refuted

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

impregnable 11 letters wn-def

incapable of being overcome, challenged or refuted

inexpugnable 12 letters wn-def

incapable of being overcome, challenged or refuted

incapable of being overcome or subdued 50% similar → invincible, unbeatable