Tiger
..is so much better on my ibook G4 1 ghz. So much. Spinning beachballs galore with 10.5, video was basically unwatchable. I haven't really found anything I can't do or can't find a sensible workaround for as compared to 10.5. I think if you've got a really fast (dual) G4 or G5 then Leopard is the logical choice. 1 ghz and slower, and a crappy video card like my ibook, then stick with Tiger. Thanks to Cameron Kaiser I have a completely up to date browser in tenfourfox. Long may he live. Flash is ridiculous of course but:
I can watch youtube (embedded) in safari in pretty damn decent quality (360p) in quicktime using viewtube/greasekit. I've got mactubes and youview for unembedded viewing and downloading. Add Coreplayer into that mix and there is very little I can't do, at any resolution below 1080p. And this from a computer made nine years ago!
Speaking of Youview the developer is no longer updating it (though he does issue fixes when youtube breaks its streaming), and instead is working on a system wide video replacer based on mplayer and quicktime he calls motionline. He's been working on it awhile, I tried to lure him out and see if he'd support PPC and Tiger (which Youview does) but he was cagey. He said its coming along but no sure release date yet. I think he'd be crazy not to build a universal binary, lots more sales from us PPC diehards that way...
Tiger is the XP of PPC mac world, it just keeps going and going and going.....