For some folks it may be the Taylor you plan to get next.
Am going to look at it a bit differently, as in maybe the guitar that helped define something. That way, a number of instruments would fit. The Liberty Tree guitars. The 912 with Cindy inlay. The 855 jumbo 12 string. Each guitar in a way helped define what a Taylor guitar is to some folks. The Liberty Tree because it had a lot of crossover press, including the cover of American Forests magazine. The 912 with Cindy inlay is in my mind the guitar that really helped establish Taylor as not just a good fingerpicking guitar, but also one that could look really nice on stage. And the 855 could be considered the 12 string that helped break the hold Guild had on the market.
With all that said, for myself the Holy Grail (or at least grail shaped) guitar would be the paisley jumbo made for Prince. In my mind it is the guitar that really got Taylor off the ground from a small struggling operation to a "legitimate" guitar builder in the eyes of many. And will also consider it a grail as it was for sale at least a couple of times. And I've held the guitar. (For those interested, it sat in a closet at The Podium in Dinkytown, MN when the shop was the bottom floor of a house).