I'd have to say the S&W 586 for the following reasons.

1. Choice of barrel length, 6'' being the best.
2. 10 shots instead of 8 from the other Umarex co2 pistols.
3. Best sights as standard of any of the Umarex range(elevation and windage plus 3 foresight blade widths) and they also can't move as they are securely screwed in place.
4. Very good trigger.
5. Barrel screws into the frame and the shroud is securely located onto the barrel muzzle thread hence the reason why the foresight can't go anywhere.
6. Easier to remove a jammed pellet due to the easy barrel removal.

Lastly when you buy a Smith, you really are buying a revolver and not an 8 shot revolver disguised as a semi auto.