Business Flyer Magazine ran an article a few months ago where they "created" what would be their ultimate business class from among the features of all the different airlines. Their choice for best lounge was the Virgin Atlantic lounge at Heathrow.
I cannot provide any first hand experience to validate Business Flyer's claim
However, of the clubs I have visited, the Delta International Business Class lounge in Atlanta was easily the best among the few I have been priveledged to sample. I snuck a peak past the door into the Delta International 1st Class Lounge at Atlanta, and it look even nicer.