It depends on the look you want. The plastic lens cameras have a certain "Polaroid look" to the photos, and that's sometimes what you are going for. If you want more sharpness, the 250 and 350 are nice cameras -- they have glass lenses and Zeiss rangefinders, and are pretty reasonably priced.

For shallow depth of field, there are a couple of ways to go. First is the 180/185/190/195 cameras, which have more ability to adjust apertures than the other cameras. These are getting pretty expensive. The other way to go is the Polaroid 600 or 600SE, which are press-style cameras manufactured for Polaroid by Mamiya. Or, you can get a Mamiya press camera with Polaroid back -- very similar but the lens mount is different.

Edit: I forgot to mention the ultimate resource on pack film cameras: The Land List.