Obviously I'm biased, but the Spectrum had the best BASIC at the time. It might not have been as fast as BBC BASIC but by hell it was a lot easier to code for. Not having to constantly look up VDU commands (for the BBC) and POKE addresses (for the C64), along with its sublime string handling made it leagues ahead of the competition.
But then, I am slightly biased. The C64 had some excellent games though.