Just one? That's a hard decision between S&W & Ruger. If you can find one, get an older 4" K frame S&W. I have a pinned and recessed 4" model 66 and it's one of the best I own. Great classic S&W styling and there increasing in value as time goes by. Also, good accuracy with .38's and .357's. Only thing about the older K Frame's is your not supposed to feed them many 125 gr .357's, they [older K frames] had problems with the forcing cone cracking with the lighter bullets. Stick with 158gr .357's and you will be good. Mine loves the 158gr American Eagles made by Federal.
That being said if you can't find a one/don't like S&W's then you cannot go wrong with a Ruger. As RantingRedneck said, 3" SP101 or 4" GP100. Ruger revolvers are built tough as nails and they will hold up for a long, long time to come.