In OS X it's
Audiofile Engineering's Wave Editor hands down.
Their sample rate conveter blows everything else away, and it's shocking how bad a job even the big names (like Logic and Final Cut) do when it comes to SRC.
This is a real eye opener:
http://src.infinitewave.ca/
Wavelab 5 Resampler (96kHz to 44.1):
Audiofile Engineering's Wave Editor (96kHz to 44.1)
