Best professional TEST CD?

Try XLO's Test & Burn-In CD, it has everything you probably need. Sweeps, reference recordings, etc. I think you can get it directly from XLO. I found this disc better overall when compared to the Stereophile disc, which I used for a long time.

To the above, I would add Chesky's "The Ultimate Demonstration Disc".

Both the MFSL Sound Check/Test cd & the Chesky Ultimate Demo Disc have been very useful during auditions and setups.