I tend to agree with Gnostic. Best prop in Oz? Doubtful. Best in the touring party? Likely.
That being said, my biggest bugbear is, much like Alexander, he plays too loose. Especially for a tighthead prop. You don't really seeing him carrying defenders in those tight runs, 2m off the ruck like Daley and Weeks did so regularly for the Reds this year. As a former team mate of mine said, on the topic of playing alongside those two last year at the Breakers, 'Daley does all the tight work you could ask for, then Slipper is amazing because he is like having another loose forward'. While I think that is great, muscling up in the tight is the biggest deficiency in the wallaby forwards at the moment.