Thats a tough question for me, truth is they are all really accurate. Some of that can probably be partially attributed to the single set trigger. I only use that feature when sighting in a rifle, but it's valuable when it comes to that. I've got a pretty fair range set up, with a target at 50 and about 125 yards.
If I had to pick one and say it was the most accurate, I would probably go with one of my 527s. I'd have to flip a coin to pick which one. My 550 is minute of tennis ball at 125 and my 557, even without the set trigger feature, is about the same.