The "best" replacement is the one that works "best" for you. Someone else's "best" may not be your "best." The simple answer is that there is not a single, "best" replacement hard drive, because there are too many factors in determining what's "best."
What are you looking for most? A fast hard drive? A spacious hard drive? Enterprise-level quality? Consumer quality? A long warranty?
Also, please post the specs of your G4 computer -- there were no less than 6 models of G4, all with different capabilities. Earlier G4 models can't take a hard drive larger than 128GB without an add-on ATA card (or hack-ish software drivers), so we'll also need to know that information before we can make any recommendations to you.