In terms of costs of living, diversity of people, nightlife (Berghain is Studio 54's wet dream (let alone Bar25), but there are also lots of operas,theaters, art exposition venues, concert halls, museums etc) Berlin is way before all other cities. But there is also so much subculture going on there, it is just amazing. Just go to Berlin and ask people why they moved there. There are ten thousands of people flying in each weekend just for recreational purposes, living in Berlin is like living in a holiday resort, just at very nice costs.
Especially Paris lost so much to Berlin in the last decade.
Paris, NYC and London may be nicer for old people, but for people at the age of a junior scientist, Berlin is superior (average preferences assumed).