The "best" OS is the subjective and can only be determined by you. It would be the OS that fits your needs the best as well as gives you the environment you need in order to get whatever it is you need to get done, done. No one but you can tell you what the "best" OS is, but we can steer you in certain directions if you greatly elaborate on what your situation is, as well as the specs of the iMac.
What you've basically asked is, "What are the BEST tires for a Ford F-150?" We don't know what size wheels you have, we don't know if it's 4-wheel-drive, and we don't know whether you live on a beach or in the snow. It's impossible to say what the "best" tire is because we know so little about what and where it will be used.
Is speed a major factor in your decision? What about compatibility? What, specifically, do you wish to use the computer for?
How much RAM does it have? What size hard drive is in there?
You can eject the CD drive by holding down the mouse button as you start up the computer, and keeping it held down until one of the two happens:
1) The CD drive ejects and all is well.
2) The Mac OS X login screen or desktop appear, in which case the procedure failed.