Macbook and Powerbook are 2 different situations, as one is an Intel machine, and one is PPC.
What type of editing do you intend to do? Anything advanced...anything from an HD camcorder that records in AVCHD?
What kind of experience do you have? You'll find that the "best" application will vary from person to person. Final Cut Studio will provide professional levels of versatility and it's VERY in depth. It also has the steepest learning curve by far.
Basically, there are 3 main levels of commonly utilized editing software for Macs (excluding 3rd party software):
-Basic/beginner: iMovie
-Amateur/Semi-pro: Final Cut Express
-Professional: Final Cut Studio
With each application, there are newer and older versions that either add or removed features. For instance, there's a big difference between iMovie 6 and iMovie 7 (aka iMovie '08). The former is more versatile, the latter is more user friendly but has a good deal less features. Also '08 deals with AVCHD, and 6 does not.
Final Cut Express 4.0 (new version) has support for AVCHD, but lacks the audio tailoring that the previous version (FCE 3.5) had.
Let us know what you'll be doing, what kind of hardware you'll be using, what types of files you'll be working with ect., and what your experience is.
Hope that helps.
-Nick