Grail is definitely a subjective term. There are only a few sets out there that I consider to be my grail sets. I'm lucky to have two of them. Both sets took years to build, and a lot of work to track them down. Lakeshore Inn, Nevada Lodge, Mapes, ACF, Starlite Cruises, Old School Binions, and maybe one or two others. But they have to pretty rare, extremely beautiful, and in playable quantities to make my list. So as amazing as they are, Tropicana 1st edition doesn't make my list because of the lack of fracs & $1s (I would need to see a full rack of fracs and 2 racks of $1s with the 5s, 25s & 100s to add them to my list). But I definitely can respect others calling sets like Olivia's, RVCL, Vineyard, or whatever else they deem as amazing to be their grail sets. To each their own.
I used to consider the Wynn Tourney set to be a grail set until we found out Steve Wynn was a pervert. Funny that changed things for me, but I still think it's one of the best looking sets of all time.