I tripped over this post just browsing around, and really got into it by breezing through every page for fear I might miss something.... Well I didn't... Whats best? Ford, Chevy, Toyota.
But I own a MK25/S600 and a MK18/G250 (older one) and a few others like Sherwood. Dude! I have been diving since 1967 and have just about tried everything and I KNOW which Regulator is the best one. (who's the best pilot you ever seen?)
It is a U.S Divers calypso Mark 4, one of the first balanced piston primaries made. Don't get me wrong, I have just for the challenge of having to make my own HP seats, kept this thing alive, but it seriously puts out air relentlessly at any depth I used it. You don't need any "special" tools to rebuild it, except to use a gauge to set the intermediate pressure. The companion secondary is all metal housing, spool, lever and and a big honkin' diaphragm that makes it breath easy without free-flowing. You can push the purge button to inflate a 50lb. lift bag faster than you can with a tire inflator on a secondary line.
My head has been poking through the teeny icebergs and you can't make it freeze up, but you can freeze both the MK25 and MK17 if the S600 doesn't beat'em to it. The G250 fairs well. I have to buy new gear from time to time since I travel/dive and you are not going to get parts for olde but goodies on the road. But tearing down the others for cleaning/inspection is a class in how to make a proprietary tool to be required along with controlling parts access, a whole Semester requirement. When they tried to make them smaller with a smaller diameter diaphragm, they had to monkey around with Venturi, spiral tubes and plastic, making them breath dry and free flow.
I got the parts for life freebie with both of the Scubapro's by the way, but I do my own maintenance. In order to save the seats with all of them, I partial dis-assemble and store. They claim the seats will not memory on the S600... Well that is not what I've seen. That's why I pop it too.