Well, they are all desirable, but adjustable-sight models are more desirable to me than the fixed-sight guys. Among the target models, increased desirability is pretty heavily correlated with scarcity.
So I guess my preferences, arranged mostly in descending order of frame size, would run like this:
Triple Lock target revolvers.
Early Masterpiece (or Masterpiece precursor) K-frames in any caliber beginning in 1940 and continuing through the five-screw guns with no or narrow barrel ribs.
I-frame .32 target models, particularly of the 1903 and Regulation Police variety.
Prewar .22/32 Kit Guns, which are just about the most generally useful and perfectly proportioned revolver that S&W ever built.
Of course I hunger for a K-32 first model from the late 1930s like the one at the top of this page, but I suspect I may wait a long time before I have a chance to add one of those to the safe.