My oldest would have to be my B&W G3, the 1st version with a 300 MHz processor. It boots up (it makes the chime sound), but since I do not have a graphics card for it, I am not sure what to do with it yet. (After all, I DID get this for free anyway.) It also came with no RAM (I can easily fill that gap) and a 40 GB hard drive. I may make it into a Mac OS 9 video-editing machine of some sort, and try out running the original iMovie or Final Cut Pro (the latter being the original version 1.0, if I can find it) on this Mac.
My second oldest would have to be my iMac G3 from mid-2000; it's the Indigo model and has a 350 MHz processor, 768 MB of RAM, a 7 GB stock hard drive (I plan to upgrade it), a slot-loading CD-ROM drive, and two dead USB 1.1 ports, which means even though it boots, I cannot use it without a keyboard or mouse! Now that I have additional hardware, I want to try and max out the old iMac and experiment with it. Any idea on how to fix the USB ports or something?
I also have a PowerMac G4 QuickSilver in my basement, that boots and works perfectly (it's connected to a large Dell CRT monitor via a DVI-to-VGA adapter, shared with the downstairs Dell desktop we use as a server; it is also used with the Apple Pro Keyboard and a spare Logitech mouse.) It has the 867MHz processor, 1 GB of RAM, an 80 GB hard drive, an HP dual-layer DVD burner with LightScribe, and a PCI USB 2.0 card added to the back. I do use it on occasion, but not often, it is considered a "backup" Mac. But it does run Mac OS X Leopard really well, along with iLife '06.
My daily use PPC machine is my 1.42 GHz G4 eMac. It is the top-of-the-line model that comes with the SuperDrive and the 160 GB hard drive. It also has an ATI Radeon 9600 graphics card with 64 MB of VRAM. I also upgraded the RAM to 1.5 GB (plan to max it out to 2 GB soon), put in an old AirPort Extreme Wi-Fi card I had lying around, and also installed the Mac Box Set (Leopard version) on it, as it only came with Mac OS X 10.4 Tiger with no additional software (not even iLife). I even got iMovie '09 running on it without much problems (via that run-iMovie-on-a-G4 hack thing). This machine even handles YouTube videos just fine, though sometimes they can stutter a bit if I am doing heavy multitasking. I also use a newer-style Apple Keyboard with it (the version with the numeric keypad) and Apple Mouse (I REALLY need to get the trackball cleaned up, as it will not scroll down right now).