Hi, the best for speed is the lowest OSX it'll run & still do what you need.
10.1.5 would run in 96MB of RAM, 10.2.x took twice as much, 10.3.x twice again, 10.4.x twice again, 10.5.x thrice again & better Video caards.
And those are minimums, twice that being fairly useful, 4 times that being pretty good.
I suspect 10.4.11 is going to be your "Sweet Spot", I wouldn't run 10.4 on PPC Macs with less than 1.5 GB of RAM, not on IntelMacs with less than 3-4GB of RAM.
OSX is very dependent on it's spped & usability by the speed & amount of free space on the HD.
So we know more about it...
At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...
Model Name: eMac
Model Identifier: PowerMac6,4
Processor Name: PowerPC G4 (1.2)
Processor Speed: 1.42 GHz
Number Of CPUs: 1
L2 Cache (per CPU): 512 KB
Memory: 2 GB
Bus Speed: 167 MHz
Boot ROM Version: 4.9.2f1
Beyond gross performance, a lot depends on the applications that you want to use.
If you have a full compliment of RAM and a good hard drive that has been zeroed lately, you should get good performance out of any OS up to 10.4.8.
From 10.4.9 to 10.4.11, quite a bit of bulk was added, as well as the demise of support for quite a few third party items.
I have long considered 10.4.8 to be one of the best OS versions for PPC and even for slower G4s.
For a stock Cube with max RAM I agree with the other advice, Tiger .8 to .11 is best for your Cube.
If you have a CPU upgrade, a more modern video card and a 7200rpm Hard Drive then you can use Leopard with good results.
You may want to visit Cubeowner.com, look around and ask that question, a friendly site for Cube owners only.
I'd seriously consider getting the RAM upto 1 to 1.5GB, here's more info on your Cube...
Here's 512MB sticks, your Cube could take 3...