For Tiger:
TenFourKit - plays Flash ok use this Flash Hack:
https://forums.macrumors.com/threads/flash-v21-0-0-216.1967041/
TenFourFox plus Greasemonkey/Viewtube/Coreplayer
You can use any videoplayer instead of Coreplayer but that's the most efficient.
I've done a guide here:
https://forums.macrumors.com/threads/integrate-coreplayer-with-tenfourfox.1970281/
and a video of it performing here:
Omniweb with a mobile phone user agent.
I've done a guide here, go to post 12 as Youtube has broke the previous methods:
https://forums.macrumors.com/threads/realplayer-vs-quicktime.1980864/
For Leopard
Use any of the above or:
Leopard Webkit playing Flash (use the hack)
Leopard Webkit plus ClickToPlugin
Leopard Webkit plus NinjaKit/Viewtube.
This is the same method as the TFF/Greasemonkey/Viewtube solution but instead using the Safari extension from here:
http://os0x.hatenablog.com/entry/20100612/1276330696
I've found this to be the best solution of them all, as you have every video format available which you can play in browser or download and play later.
All the suggestions for Tiger I've used successfully on a 1.33Ghz iBook and Powerbook, the Leopard ones on a 1.67Ghz Powerbook.
Higher machines wont have any problems - my G5 Quad happily plays Youtube with Flash at 1080P.