World Cities
So, it appears that one of (many) attempts to define and categorize the notion of a 'World City' is from the Globalization and World Cities Study Group at Loughborough Univirsity. Their 2008 study lists 12 classes of world city, from Alpha++ (the most important category, comprising just New York and London) down to Sufficiency, the lowest level of city that they categorize.
The list is here. I think I've visited at least one city in every category (two or more in many catogories). The only category I don't have 100% confidence in is Gamma
My personal World City list
Alpha++: London
Alpha+: Paris and maybe Milan
Alpha: Torronto, Brussels, Mumbai
Alpha-: Amsterdam, Rome, Vienna
Beta+: Washington, San Francisco, New Delhi
Beta: Hamburg, Bangalore, Boston
Beta-: Munich, Dusseldorf
Gamma+: Chennai, Stuttgart
Gamma: Ljubljana (I think)
Gamma-: Manchester, Edinburgh
High sufficiency: Glasgow, Leeds, Pitsburgh, Bristol
Sufficiency: San Jose (CA), Pune, Newcastle, Reykjavik
The list is here. I think I've visited at least one city in every category (two or more in many catogories). The only category I don't have 100% confidence in is Gamma
My personal World City list
Alpha++: London
Alpha+: Paris and maybe Milan
Alpha: Torronto, Brussels, Mumbai
Alpha-: Amsterdam, Rome, Vienna
Beta+: Washington, San Francisco, New Delhi
Beta: Hamburg, Bangalore, Boston
Beta-: Munich, Dusseldorf
Gamma+: Chennai, Stuttgart
Gamma: Ljubljana (I think)
Gamma-: Manchester, Edinburgh
High sufficiency: Glasgow, Leeds, Pitsburgh, Bristol
Sufficiency: San Jose (CA), Pune, Newcastle, Reykjavik