"Best" depends on what you're looking for in post-processing software, but a few of the more popular open source packages you could look at using would be:
The GIMP with dcraw. The GIMP is the open-source alternative to Photoshop, and has a very deep and sophisticated feature set, with quite a bit more control than you could find in Picasa. Tutorials are plentiful out on the interwebz. dcraw adds raw capability to the Gimp, so you do need to use them in conjunction.
If, however, you'd prefer something more workflow-oriented that looks more like a RAW conversion tool such as Lightroom, vs. Photoshop, then three open source packages to consider are RAWTherapee, darktable (no Windows version, only Linux/OSX), and photivo. These types of editors are non-destructive (i.e., the original RAW file is left intact, and edits are saved separately. Upon export, the instructions in the edits are then applied to the original file--you always have access to all your original image data), and are generally easier to learn than a tool like the Gimp or Photoshop. They may be more feature-limited, but they have much more editing capability than Picasa or iPhoto.