I'm going to go out on a limb and say the next 'holy grail' show will pop up sooner than we all expect. Just like Jonas said, we've had stuff fall into our laps totally unexpectedly over the years.
Mountain View 04 was obtained at random when the taper contacted Mark and offered it to him.
Kings College, Palladium 01 (as well as She Couldn't and Plaster demos) popped up on eBay.
Camden 04 was totally unknown to us only a short time before we released it.
MFR was thought to have possibly been lost before we got it.
The London workprint was a surprise addition in trying to obtain MFR.
As far as I'm concerned, the only footage at this point which has 'holy grail' status is the Xero show, and that's not even 100% confirmed/documented. No offense to anyone's want lists in this thread, but very few audience sourced shows are going to have the quality to live up to the hype of really wanting it, except in cases where it's a show that the person wanting it attended that particular show.
After Xero, those that I might consider for such status:
The Ostrick tapes (though the owner seems to want enough $ for a down payment on a house for the tapes).
Smokeout 03
The Roxy/One Step Closer video taping
Mexico 02 (which we THOUGHT we were getting. Not even going there...)
After that... Things like Rock am Ring 07 and similar shows are unlikely to be obtained (by us, mind you) because they're being held by the copyright owner or withheld at the band's/label's wishes. Those above are more likely to end up in our possession.