NA - definately Boston - maybe Chicago and SFO
SA - no experience
EU - York, Paris, Rome
Asia - Tokyo for sure - Hanoi, Kuala Lumpur
Africa - no experience, but from what research I've done for my own trips, Cape Town and Nairobi - and Lagos if you are adventurous
Pacific - no experience - but friends say Sydney and Wellington are great
Middle East - Jerusalem, Amman, Muscat