Nearly all of them are good when money is not a factor.

Guys tend to avoid Accra due to lack of things to do there/ climate/ tropical disease potential. Sao Paulo is often avoided due to the hours at which the flight operates. Some guys avoid London due to the unfavorable exchange rate, as well as the layover location (not in London).

Beyond that, it comes down to personal preference. They all have something to offer, and the hotels are generally very good. I haven't seen a bad layover yet.

My favorites: Rome, Berlin, Barcelona, Budapest, Kiev, Venice. Moscow is interesting once in a while.