Which version of iTunes is best?

There's absolutely no sound difference between versions of iTunes. Sometimes the updates will have new features, but usually they address bug fixes, security updates, and DRM changes.

Short answer: the latest varient!

Long answer: In actuality, there have been slight variations in the sound of iTunes. Some of the updates (but not all by any means) actually added a slight improvement in high frequency reproduction and better harmonic texture throughout the entire frequency range. Nothing major mind you but better for sure. The best part of iTunes is that the LATEST offerings seem to always be the best sounding i.e. just make sure you have the latest variant.

BTW: I should state that the above improvements in iTunes are not necessarily evident unless you are using a USB connected, non-SPIDF DAC with serious high end electronics, very transparent speakers and premium USB cabling i.e. SPDIF connectivity and/or budget minded gear downward in the system chain could absolutely marginalize sonic differences between all of the iTunes variations.

Thanks for your great responses. I have learned a great deal from all the threads here on Audiogon.