Best - London (but expensive). I don't know what people bother debating it.

Dubai are also very good. The drivers may not be very knowledgeable but taxis are everywhere and very cheap (cheap labour, no taxes, cheap fuel)

Worst - Boston. Ancient cars, typically held together by rust and gum. Most of the drivers seem to have only arrived in town a few days before and haven't a clue how to find their way around.

Most frightening - Mumbai. Taxi driver started driving in the wrong direction down narrow dark streets insisting that I meet his brother.