I own self-cocking doubles by 11 different makers, 4 different nationalities. The least noticeable that comes to my mind, as far as cocking effort and smoothness, is the Beretta 687 SPII/30". Barrel length plays a big part in noticeable effort with cock-on-opening guns. The longer the barrels, the more gravity helps you. The Perazzi is very smooth, but the barrels are still fitted so well to the receiver that the only real noticeable effort there is friction within the receiver/barrel relationship.
I have several Foxes, all are smooth, with none of the feeling of parts rubbing together, as you mentioned. My Dickinson is hard to close, but opens and cocks the hammers easily. The closing difficulty is from very strong springs in the ejectors. There was a thread awhile back about someone who made some adjustments to that by cutting some coils off the springs. It lessens the throw of the empties, but makes for easier closing. The ejectors are excessively strong, IMO. The only Purdey I ever shot was very smooth opening and closing. Terry Allen, the brilliant gun photographer, had a little Grant, as I recall, a few years ago that was a cock-on-closing double. It was so smooth and well built that it was hardly noticeable. I saw him showing it to a couple guys and asked him if it was cock-on-closing gun. He looked sheepish and replied, "Well, I don't know". He pulled the triggers, opened it and closed it, and still didn't know. He handed it to me and I did the same, but felt slightly more effort was necessary to close it than to open it, indicating it was cocking on closing. But, it was very smooth.
I don't recall ever noticing any real "roughness" when cocking any of my guns, really. But, now that you have "breeched" the subject, I just may have to go to the safe one day soon, get them all out, and do a comparison check.
Interesting thread, CZ.
SRH
