OP- I'm pretty sure that the Walker is the most powerful BP pistol revolver ever made, i.e. it has the largest powder capacity, and was the "magnum" of its day.
The Dragoons were basically modified Walkers, designed to make the guns lighter/quicker/easier to point and shoot. A Dragoon is a Walker that was put on a diet.
There's no answer to your question- because the only "correct" answer for us BP addicts is- you have to buy all of them. Get the Walker, and 1st-2nd-3rd model Dragoons !
But if I had to choose just one, I'd get the Walker first. It's heavy though.