You currently have a N-frame and two J-frame S&W revolvers. So obviously, you need a K-frame to go along with them.
You can't do better than a Model 18. Its my favorite revolver. The K size frame is ideal for the best accuracy and feel. While the 4" barrel is a great all around length for everything from casual plinking to small game.
They're getting hard to find as those that have 'em tend to keep 'em. Nor are they cheap. But if you get one, you'll never regret it and keep it forever.