My favorites are my Ruger Redhawk, 7.5" in 44 Magnum, Ruger Mk. II Target, and my Bersa Model 383 in 380 ACP.
I hand load, so neither round is as expensive as you would think by looking at the prices for factory ammo.
For 44 Magnum, I load up 44 Special level loads in the 44 Magnum cases. Super accurate and easy to shoot well. They don't beat me up, or break the wallet. (the magnum loads use 3X as much powder as the Specials!)
The Bersa is just amazingly accurate for such a small gun. I shoot it about as well as I do my full size 1911. It must have something to do with the barrel being fixed. It is also a compact size, but without feeling cramped.
The Ruger Mk. II is just deadly accurate. Light recoil, and a great feel in the hand. Cheap ammo is the icing on the cake.
Now, I don't want to be one of those guys who doesn't pick just one when he's asked to. So if if I have to pick just one, I will pick the Bersa. With a little guilt. It is not as accurate as the Redhawk or Mk. II, (hence the guilt) but it just makes me feel so .... competent. I feel like if I got into a real gunfight, this is the one that would give me the best fighting chance. Easily controllable recoil, good accuracy, enough oomph, quick to reload, and not too big to actually carry.