For me, it would be the 6.5mm JDJ. While the Contender was already popular, the introduction of the 6.5 JDJ -- and to a lesser degree the .375 JDJ -- helped put the Contender format into the limelight, so to speak. It may not be an original factory chambering, but the words "6.5 JDJ" and "Contender" go hand--in-hand like peanut butter and jelly ha ha. Even with the lack of new .225 brass, the 6.5 mm JDJ remains one of the most coveted Contender chamberings.