I can't speak for USD, but I did some research for EUR, GBP and SGD. For those currencies my preferences are:

EUR: Kraken (great exchange if you're European, not such much for Americans)
GBP: Cryptopay (accepts EUR and GBP; very good rates, though a 1% fee applies)
SGD: itBit (free SGD withdrawal and dirt-cheap deposit, if you happen to have a Singaporean bank account...)