This is a good question, but I imagine you'll get as many different answers as there are forum members -- especially if we're going to throw tone into the equation, which is such a subjective matter...
To me, for something to be considered a "holy grail" it would have to be unique, desirable (collectible) for one or more reasons, and rather difficult to obtain due to limited numbers produced. Given that premise, and the relatively limited time in which Taylor has been producing guitars (compared to Martin), I'll put forth
the Liberty Tree T5. There were only 50 made (in fact, it's considered Taylor's second "Builder's Reserve" guitar); each one is unique in that they were numbered and named for each of the 50 states, in the order in which that state entered the Union; they all got scooped up immediately; and they just don't come up for sale very often -- and never at anything resembling a reasonable price...
I would venture a guess that the first thirteen (Delaware, Pennsylvania, New Jersey, Georgia, Connecticut, Massachusetts, Maryland, South Carolina, New Hampshire, Virginia, New York, North Carolina, and Rhode Island) are the most desirable units in the series; and given the source of the wood used in these guitars (the last-surviving Liberty Tree that was in Annapolis, MD), #7 (Maryland) is probably the most desirable among those...
But which is considered the
best-sounding Taylor...? Whew! Good luck getting a consensus on
that question...! We all can't seem to agree whether the the recent bracing changes were good or bad...
I'd generally say any Taylor made with Brazilian Rosewood; but not even all of us agree on that premise...