GP 100. inexpensive, easy to find and durable. S&W 28 would be nice but the prices are rising to the level of the 27. I did see a 6" blue 27 this weekend at $400, very good price especially in CA, though about it, but my dad has a 90% one already so I passed.
27's in 3 1/2" while potentially more common than a 4" although the 3 1/2" was discontinued in the late 1970's seem to be more desireable and fetch a higher price than 4" models. I would have bought the aforementioned model 27 if it would have been a 3 1/2" model.
Another good gun that has also been rising in price but are a good find would be a Ruger security, speed or service six. Roughly the same size as a S&W K frame 357 mag, but much more durable.