Phrases like "the Holy Grail of Physics", are snowclones of the form "Z is the X of Y". They work because X's properties are well-understood and can be used to immediately relate Z and Y.
So if someone says
Artificial Intelligence is the Holy Grail of Computer Science
Then everyone knows what that means: AI is something that is rumoured to exist (or be possible to create) and it is as fervently desired to computer scientists as the real Holy Grail would be to religious people/Indiana Jones.
The point of all of this is that it doesn't matter what X is; all that matters is that when "the X of Y" is put together, you understand what the comparison is.
The Switzerland of Africa
The Elvis of hip hop
The reason I say all of this in answer to your question is that "The Holy Grail of Y" doesn't have religious connotation, and it's the best phrase to use. It's a mythical object that has been the source of quests to discover its location, etc. It has inspired many fictional tales. People dream of finding it. There is scant reason to believe it could actually exist, but it is plausible. Its origin is in literature, not religious practice. So use "the Holy Grail of Y", just like you freely say "Good-bye" without worrying about its etymology.
A further reason to use this phrase is because it only works when people get your reference. Example: bib's comment below, where his father calls himself "the Derek Jeter of mussels", was completely opaque to me because I have no idea who Derek Jeter is. If you find a synonym, such as Andrew Leach's "El Dorado", which actually suits your meaning, you have to be careful that it is obvious why you are making the connection. Everyone knows what the Holy Grail of Y is. People might not understand what the El Dorado of Y is.