MK 10 Balanced Adjustable.
The Mk 10 is simple, realiable and delivers all the air you 'll ever need. The silicone filled SPEC kit also allows it to be used in extreme cold with no concern for freeze up. It is smaller and ligter than the SP MK 15, 20 or 25 and is cheaper and easier to service.
The Balanced Adjustable is the immediate ancestor to the G250 and shares the same internal design and parts. Breathing performance is equal to the G250. The only significant differences are the lack of a VIVA assist switch and a brass case rather than a graphite/phenolic resin case. The fixed VIVA assit on the Balanced Ajustable is just right to provide minimal breathing resistance without a positive pressure/over pressure phase to the breathing cycle.
You can also drop a tank on the second stage, pound out the bigger dents and go diving. It is super tough and the chrome finish does not look like crap after a year or so of use like a graphite or resin cased reg.
It is also better suited to extremely cold water diving as the metal case offers superior heat transfer.
In my opinion, the Mk 10 Balanced Adjustable is where regulator technology reached it's current perfromance plateau. Since then, the only real "improvements" have been in reduced manufacturing costs, (at the cost of durability) smaller sized second stages (at times at the cost of perfromance) and styling.