What is the most powerful Mac capable of running Mac OS 9?

I know that there was a period of time after Mac OS X came out that new Macs still shipped with Mac OS 9 for a number of reasons (OS X was buggy, better compatibility with Classic apps, etc). This is more of a curiosity than anything, but I'm wondering what the most powerful Mac capable of running Mac OS 9 is, and what the maximum specs are.

Solution 1:

According to Wikipedia, the Power Mac G4 (Mirrored Drive Doors 2003) is the newest and most powerful computer that can run Mac OS 9.

1.25 GHz PowerPC G4 (7455/7455B); can hold up to 2 GB memory (although OS 9 might not recognize all 2 GB).

Solution 2:

There is a slight possibility for misunderstanding in the question. If the question is meant to ask for the most powerful 'true' PPC-Mac as shipped by Apple and with native OS 9 installed or installable, then Daniel's answer is correct.

However there are other options eligible to answer the question as is. This involves the cheating option of citing first later Macs, including G5s, that ran OS 9 as the Classic Environment, or even later models emulating OS 9. Later G5s were extremely fast at running OS 9 and its applications.

According to enthusiasts there is a second, a less cheating answer that involves upgrades. You could swap the daughter cards of later G4 models into the last native OS 9 machine and enjoy a faster machine. You can also opt for one of the aftermarket options from e.g. Sonnet or Newertech. These are hard to find today. Remnants of their specs are rarely found on the web even if the companies that developed or sold them still exist. Finding the on sale is even more difficult.

Newertech even sold an upgrade with a G4 chip that Apple never sold: Apple stopped at a beefed up 7447 at 1.67 GHz and 7455 at 1,25 GHz while NewerTech opted for the 7448 in its MAXPowerG4 cards. This came to market after Apple flicked the kill switch for PPC for he transition to x86 processors. For that some benchmarks and some specs are still on the vendors site. For a general overview compare the list here at lowendmac.com.

Therefore the most powerful true Mac booting natively into OS 9 has to be something like a Power Macintosh G4 'QuickSilver 2002' upgraded with MAXPowerG4 with a single NewerTech G4/2.0GHz 7448 MAXPower G4 or a NewerTech DUAL G4/1.8GHz 7448 MAXPower G4.

Depending on the usage scenario dual or single is a tough choice speed wise since OS 9 can not take that much of an advantage of two threads. Although the bus speed might hamper the final results quite a bit there are still more options to consider for "the most powerful": these things had a few standard PCI and AGP slots. So the graphics card could be upgraded significantly beyond what Apple ever offered and even SSDs could make a bit of an impression considering their latency compared to magnetic drives.