Best Linux flavor for iMac G3/333?

I've heard that OpenSuse is good, but I'm not sure on such a low end G3 imac

i would have to say that gentoo is the best choice, not just for this computer, but for any. The package management system is the best you can find, everything is compiled specifically for your system, and you have total control over how your system operates. The only downside is that it takes some time to get up and running and is a more advanced install.

for you old mac, Xubuntu is the best mainstream distro
1. its light weight, so it runs faster on older machine than those big distros like SUSE/Ubuntu/Fedora Core etc.
2. XFCE is a decent desktop environment
3. It came from famous Unbuntu family, so you can be sure it easy to setup and use.

Gentoo is hard to setup, recommend only if you know alot of linux
SUSE/Ubuntu/Kubuntu/FC/Debian are too big, won't run smooth on your machine
Damn small linux etc. ultra small distros probably are too poor to use for you

I remembered read something say, not very sure, but you get the idea.
KDE need 512Ram, Gnome need 384Ram, XFCE need 128 ram to run smoothly.

Best distro

No doubt that the best option is ubuntu linux.

Xubuntu!

Debian?