Most on the S&W Forum consider the 686-4 to be the best. Last version with Forged Parts, and Traditional Hammer Mounted Firing Pin.
I have a 686-1 and a 686-5 Both are Power Custom Combats
I like them both very well.
The wife took over my 686-1 as her House Gun.
The 686-5 is the first one with Floating Firing Pin, and MIM Parts. The Cylinder Std was replaced with a much improved design. It has a pinned front sight. It is also the last without the LOC. My 686-5 is my SSR gun and it to me is about as good as it gets. Ron Power worked on it, and it runs like a Swiss Watch.
You will more than likely really like which ever one you end up with just fine.
As far as I am concerned their are No Bad 686's.
The Perfect 686 for me would be a 686-4 with 3" Barrel in a 6 Shooter. That is my Holy Grail Revolver at the moment.