Imperial Treasury Vienna
Why? Because only there you can admire the crown of the emperors of the Holy Roman Empire or the Agate bowl that was belived to be the Holy Grail.
If you like "treasures", emporerstories, invaluable relicts and items, you really have to go there.
With over 1000 years worth of Royal treasury items the Imperial Treasury (Schatzkammer) at the Hofburg Palace is regarded as one of the most important in the world.