I think there have been a lot of too obvious reponses. With people confusing awesome records with great DJ'ing (Butch etc).
A good DJ doesn't necessarily have to have the best records. Butch has the best records (these days anyway) and is a competent DJ, but not the best.
The definition of a truly great DJ is one who reads his crowd perfectly and plays to entertain those that are there. Amongst the best DJ's I have witnessed are (in no particular order) - Ian Levine - Mecca times (could have been a real contender ), Richard Searling, Dave Withers, Pat Brady, Steve Whittle - Casino days, Mark Bicknell (when he's not tired & emotional ) and Dave Evison.
Phil