Probably the GinnerPotter bg vids before she started selling them regularly and before that Minigxo IMO.
For a vid to be grail status it's gotta be:
1) a real video not something you wish existed ie there's caps or small clips etc
2) rare as fuck ie not sold or advertised maybe it's a reward for a raffle or high tip contest
3) legit porn of a hot model where she's doing something she usually NEVER does ie b/g content
Yeah, I'll go along with this definition of a holy grail video. Most of the stuff mentioned in this thread doesn't qualify. I'd even add videos that there are only
rumors of existing. I'd say those are ones that a few people paid ALOT for and will never see the light of day.