The IAU Central Bureau has compiled a list of the Top 20 Closest Approaches to Earth by Comets. Comet Hyakutake B2's expected 0.1019 AU flyby on March 25, 1996 will rank it at number 19. To give you an idea how rare Comet Hyakutake's near-Earth flyby is, there has been only 4 closer comet flybys in this century, the last being Comet Sugano-Saigusa-Fukikawa (0.0628 AU) and Comet IRAS-Araki-Alcock (0.0312 AU), both in 1983. The record for the closest comet flyby goes to Comet Lexell which missed the Earth by only 0.0151 AU (1.4 million miles) in 1770.
Comet 1996 B2 Hyakutake Home Page