This is a very subjective question, and I'd have to experience ATC in every country of the world to give a worth-while opinion. Having said that, the best controllers are those who seem to have a GOOD appreciation of what it takes to operate an aircraft from the pilot's stand-point, leading to a degree of flexibility that is required when operating aircraft, and if that be so, the Brits and the Australians win hands down.
The Americans are good, damned good, but seem to be either far too rigid, or at the opposite extreme of far too "do it any damned way you want", which worries me.
If flexibility and rigidity resulting from a good understanding of what it takes to operate an aircraft, and rigidly enforce their own requirements with little thought for operational implications, the Indians win as the worst. (Guess who has an up-coming tour of duty in India?).
Regards,
Old Smokey