-6 is when S&W started putting the lock on the 686; -5 and earlier are all pre-lock, and -4 and earlier are all pre-MIM hammer/trigger (-4s had a MIM thumbpiece).

As to actual configuration, I would consider the -4 to be the best 686, with its factory drilled & tapped frame to accommodate a scope mount and optional 7-shot cylinder. -3s and earlier were not drilled & tapped at the factory and did not have the optional 7-shot cylinder. Now all I've gotta do is find one and buy it, but I have it's functionality replicated in my two 686s (a -1M and a -5).