I own a 686-5 (7-shot) with a 4" barrel and a 686-4 (6-shot) with a 2.5" barrel. I've also spent some extensive range time with my father-in-law's 686-6 (6-shot) with a 6" barrel.
They are all superb guns, but if I had to pick one as my favorite all around gun it would be the 4".
The 6" with the full under-lug feels just a little bit too barrel-heavy for my taste, and the 2.5", while real fun to shoot, is just a little too short to make it a good plinking gun (at least with my shooting ability). I find the one in the middle to be "just right." I think the 7-round cylinder is a real bonus too.
That being said, I have my eye on a 6"686 just to round out my collection.