tough one!
The first time I saw (hed) was during OIA, pre B2BX. They just got signed to subnoize. I think that was the best one yet.
But Anthrax in Paard v Troje, The Hague was also great. though that was with the vocalist they kicked (Dan Nelson).
Metallica, in Amsterdam a couple of years back. Also was pretty fucking awesome.
Man I could go on for ages, and still be in doubt about which one was the best ever.