what are the "must have" CD's

For years, I've invested in quality sound equipment and have a small collection of CD's of different types.....but I realize that there are problaby some "must have" CD's that really show just how good (or bad) a system really is. This isn't a quest for who your favorite female singer is...or what orchestra you like the best....I'm really after sonic quality and sonic range (gotta show off the sub and boomy rock probably isn't the best way to do it).

I'm open to suggestions in most any musical category...the only qualification is that the CD has to be readily available and not some collector's item.

If there is already a list somewhere..please point me to it....or post your suggestions.