In terms of the occasion Spurs or West Ham by far. Proper Derbies. The atmosphere is electric when we come to town and you won't hear anything like it unless you go abroad.
Eastlands for me is the best by a mile though.
I really like the Emirates (Library). Modern yet unlike Old Trafford still feels like a football ground, obviously the higher up you are the better.
Villa Park is decent as well, as is SOL
Best away support I've heard at the Bridge would probably be Barcelona or Atletico Madrid, in this country probably Forest.
I'd say the best away support in the country in terms of numbers is easily United but vocally I think West Ham are begrudgingly as good as any I've heard. Lost respect for Spurs when they abandoned their team at Wembley