A tough one. I love all my PPCs for different reasons. A bit hard to say what's more beautiful out of a e.g. QS G4 and a Titanium G4, as the form factors are so different. I suppose if I could only keep one, it would be the Cube, as it's so unique and looks good on a shelf. OTOH, the Titanium also looks great, is portable, and is usable as both an OS 9 and OS X machine (especially the 1GHz model), making it everything you'd need in one compact retro Mac.
The iMac G4 is still a fantastic piece of design, though its lead-alloy base makes it surprisingly heavy (at least the 20" is), so not quite as convenient to put on a shelf. The Pismo is also cool in its own way. Very thick by todays standards, but ergonomic and so expandable.
The only machines I'd still be interested in buying are a TAM (though pure unobtainium, especially for any money I'd pay), and a 12" G4 Aluminium. The problem with laptops is the batteries, which are always dead, and hard to come by / usually expensive, though you can get lucky.
I'm not really interested in any of the later PPCs e.g. G5's, 15" PB G4s, as the Intel versions look just as good, have more modern interfaces / ports (e.g. MagSafe), and can run modern OS's, especially with an SSD. They're also not that much more expensive. And in turn, those are too new to be a novelty. Also, if we're talking Intel laptops, I prefer the unibodies, for being much easier to work on, as well as more rigid. I have a 2012 13" MBA, and aside from the bezels, is pretty much the perfect laptop (in terms of value and lightness).