I have a few definitions for "oldest"...
Power Macintosh 7100/66 - the very first PowerPC Mac (released at the same time as the 6100/60 and 8100/80.) Released March 1994, this model can boot System 7.1. The line was speed bumped (6100/66, 7100/80, 8100/100) in January 1995, and those models require System 7.5. My 7100 is the "base" model, without even a CD-ROM drive. I have it paired with the 'matching' Apple AudioVision 14 Display - the only display to use the HDI-45 connector that was only present on the x100-series Power Macs. I also have an Apple Adjustable Keyboard for it.
Power Macintosh 6100/60AV - released the same day as the 7100 above, this is the "pizzabox" form factor, and this one has the full AV in/out ports plus CD-ROM drive.
Those two, simply, are the first PowerPC Macintoshes Apple introduced. But I have a few more 'interesting' ones:
PowerBook 5300: The first PowerPC portable - I have a few of these. One grayscale 5300 at minimum specs, a few 5300c with color display, and one top-of-the-line 5300ce (which had a higher-resolution color display and a faster CPU,) but all of them are broken in some way. They all boot to an OS, but something is wrong with each of them. (the 5300ce has a completely broken display, sadly.)
PowerBook 540c with PowerPC Upgrade - the PowerBook 500-series was introduced a month and a half after the desktop x100-series Power Macintoshes, with the promise that Apple would release a PowerPC upgrade. I have one with the PPC upgrade. I also have a 520c with the stock '040 chip.
Quadra 700 with Apple PowerPC Upgrade Card - released in 1991, well before the PowerPC line, with no promise of upgrade (Apple and IBM didn't even sign the deal that would create the PowerPC processor until the week the Quadra 700 was released!)
Apple later released a PowerPC Upgrade card that plugs in to the PDS. Sadly, my PPC card for this is dead. (I also have a Daystar PPC upgrade that works, which is essentially the same card, since Daystar made the cards for Apple.)
I also have an IBM PC Power Edition 440 - IBM's attempt at a low-end PowerPC desktop, released about the same time as Apple's x100-series, designed to run Windows NT, Solaris, and OS/2; later they allowed AIX to run on it, and neither Solaris nor OS/2 were ever released for PowerPC in a truly usable form. The line died quickly, but the similarly-architectured AIX-only RS/6000 line continued just fine.
Finally, I have an IBM ThinkPad 820 - a PowerPC ThinkPad roughly feature-equivalent to the PowerBook 5300c, and released at near the same time. Much like the "PC Power Edition" line, IBM's PowerPC ThinkPads didn't last very long. (The last one was rebranded as an RS/6000 laptop and sold for a few more years.)