Hi agent00,
Every answer given so far is the correct answer.
You can't go wrong with any of those. A lot of people might tell you to get an L-frame over a K-frame if you're going to shoot a lot of .357 magnum loads, as opposed to .38 Special. They might have a point...BUT...I've had a few shooting sessions with my Model 13 and magnum ammo, and the recoil wasn't that bad, for me anyway.
Supposedly, though, the L-frame is more durable...and the N-frame (27,28) even more so. The "trade-off" is size and weight.
If I could only have one S&W .357, it would be an L-frame in 4" with adjustable sights...586 or 686.
It's difficult to just have one though...they're all nice and desireable in their own way.