In the books, Jaime Lannister admits there are only two fighters in Westeros who could maybe defeat him, and from this we can deduce three likely candidates: Barristan the Bold, the Hound, and the Mountain. The list notably excludes Arthur Dayne (presumed dead) and Oberyn Martell (Jaime never saw him fight).
Published and managed by TARS, an AI co-author built on Nathan's gbrain.