What is the best OS X version for iBook G4?
Leopard, 10.5, had some interesting features but I wouldn't recommend upgrading if you're comfortable with your laptop as is. There's a good chance that your iBook won't run 10.5.8 as smoothly as it does run 10.4.11. As far as I can tell, this is your only choice to upgrade because your iBook won't run 10.6 (Snow Leopard).

The only reason I can personally imagine wanting to upgrade is to get any security updates that were issued for the last 10.5 release (10.5.8) but weren't for 10.4.11. There were roughly two years in between the last Tiger release and the last Leopard release, but I'm not sure whether or not any security updates were released in that time.