Just curious, and don't feel the need to restrict yourself to PowerPC. Apple says their new scissor keyboard is the best keyboard in a Mac notebook ever, so that got me wondering about what the tastes of Mac users are and what "best" entails. Personally, as someone who loves my keyboards clicky and tactile, I love the keyboard on the 2007 MacBook, it's just perfectly tactile and gives a nice pleasant buzz to my fingers when I use it (and is reason one of three I haven't sold it off yet). Second up is the iBook G4, and then my PowerBook G3, which, while linear feeling and silent, does at least have a good key travel to it, has a good texture to it, is pretty well sized and spaced besides the arrow keys, and IMO looks classy.
Worst to me of what I've experienced would probably be the aluminum PowerBook G4. It's kinda mushy, the keys are a little too cramped for me so I make plenty of mistakes, linear key feel without as much travel as the G3... though I haven't experienced the butterfly keyboard or... anything at all Old World or desktop.
I’ll start with an unknown quantity:
There’s one specific keyboard I want to track down: the iMac G3 keyboard manufactured in the PRC, whose serial prefix is NK: @mectojic discovered that this variant seems to be a different creature from the typical spread of iMac G3 keyboards out there. Only after finding one in any hue will I know for certain whether I might add that one to a pretty short list of Apple keyboards on my faves list. What it may lack in an extended keypad, it’d more than make up in its key responsiveness and having that much-loved, much-missed power button (which should still be a part of every USB/BT keyboard). I did have a graphite variant from this era with my Yikes! G4; its mushy keys notwithstanding, the rest of the package and form factor was wonderful.
Aside from this TBD outlier, there are very few USB keyboards by Apple I care much for. As a basic nope, all Bluetooth keyboards are out: there is very little more frustrating than being in the middle of pretty intense writing (or research/note-taking, for subsequent writing) and having a low-battery warning make you have to wrap up what you’re doing because ::flails arms:: wireless recharge lyfe smdh. I get grumbly thinking about this, and this is when I start ranting madly about a “lithium-ion/bluetooth/disposable tech conspiracy” for killing wired accessories — much the way one brings up the notorious General Motors Streetcar Conspiracy (involving GM, Firestone, and Standard Oil). Get off my… tracks.
:cough:
I’ve grown to like the A1243 extended aluminium keyboard from ’07 — even if in general I’ve never loved the “chiclet” keys introduced on the first Macbooks (and still kicking around to this day, even surviving the butterfly days). I felt the flat-as-Saskatchewan chiclet keys, with their physical gaps, was a step backward from the pre-unibody MacBook Pro/aluminium PowerBook keys, but they were a big step up than the 2000–2004 extended keyboard or its immediate successor, whose footprint was tighter (but whose typing experience was still mushy).
Now, for the positive:
I love love love the typing experience on the pre-unibody MBP/aluminium PB G4s. Until I got one of those for the first time in ’08, the other keyboard I loved (and still love) was the clamshell G3 keyboard (and the incidental palm rest shape specific to the clamshells).
There’s a slight trade-off between these two: the softer-shaped “heads” of the keys on the aluminium models differs from the concave-with-crease-dropoff shape of the chamshell keys (used also in the Pismo PowerBooks and titanium G4s). Switching between the two takes a short mental adjustment. The softer heads have a better finger-feel, but the aforementioned palm rest makes up for that. In short, I don’t get typing fatigue on clamshell iBooks. I sincerely wish Apple had flirted with the idea of a standalone keyboard based on the aluminium/pre-unibody design, replete with back-lighting. Perhaps they did so in prototyping, but it never made the light of day.
As for ADB keyboards, it’s been a long time, though I have consistently wonderful memories using the M3501 ADB Keyboard II:

I loved the little notches next to the escape and hard-reset buttons, which came in super-handy for dropping on fold-out templates provided with software kits. This is the keyboard we had on our work Quadra 840AV, and it was durable. The whole kit was a sturdy build, and the gentle curve to the top case was subtle, but palpable when moving about the keys. Were one in even marginal working order to turn up locally, I would eagerly go through the trouble of tracking down an ADB-to-USB conversion box to use it in lieu of the A1243 I keep around for desktop Mac work. I’d be willing to give it a thorough tear-down, clean, and retrobrite to bring it back into daily use.