Before I begin, I want to say that the following is my opinion and mine only. Please feel free to disagree, and I have no doubt that many of you will.
First, I view all Daytona models before 1989 (ie. pre-16520) as more functional watches, neither aesthetically beautiful nor status symbols. This all changed in 1989 with the release of the ref. 16520; for the first time the Daytona (at least the SS version) was in high demand, for the simple reason that it simply looked stunning to the eyes. The design of the 16520 combined elements of the vintage era with those of the modern and is to many the bridge between the past and the present, offering the best of both worlds.
To me, one of the criteria for being a "grail" is, it has to stand on its own as aesthetically beautiful, not just because it is rare or was worn by a celebrity. Which is why I've ruled out all pre-1989 models as contenders (I know I know, the hate mails are being generated right about now). So the real question for me is, what makes the 16519 better than the 16520 and all subsequent Daytona models up to the present? The following are several factors to consider:
1) The 16519 was only available with a strap, and this takes away one major problem faced by many Daytona owners: the bracelet / end-links becoming loose. When this happens, not only does it look bad, loose end-links will scratch against the case, and the bracelet will (when it loosens to a certain point) eventually scratch the lugs as well. For those who try to avoid this dilemma by fitting their bracelet Daytona with a strap, an aesthetic flaw quickly emerges: the space that appears between the lugs (a flaw that is non-existent in strap Daytona models like the 16519).
2) This 16519 is in white gold, but looks no different than steel. The heftier feel of precious metal is, in my opinion, an advantage over the lighter SS.
3) The 16519, especially in this dial configuration, is far more rare than the 16520, 116520, and 116500LN (and probably more rare than most vintage Paul Newman variations). The fact that this is a "P" serial (with luminova dial) is cherry on top of a grail sundae.
4) The 16519 has the shiny and rounder lugs of the 116520 / 116500LN. However, I feel that the lugs of all modern Daytona's are a tad bit short. The 16519 takes care of this problem because its lugs, although shiny and rounded (and modern-looking), are longer like those of the 16520, but without the sharp edges. Another example of the 16519 combining the best of the past and the present.
5) The 16519 basically has a modern clasp design that is sturdier. While not an advantage over current Daytona's, it is certainly better than the thin, light, and stamped clasp of the 16520 and anything prior.
6) I feel (as many do) that the subdials on the 116520 / 116500LN sit too high on the dial. They are clearly more centered on the 16520 / 16519. It may be a simple matter of preference or, some might say, geometry. This pretty much takes out of contention any modern Daytona up to the present, including but not limited to the 116519, 116519LN, 116515LN...etc.
7) Like the 16520, the 16519 is powered by a column wheel horizontal clutch movement highly modified from the Zenith El Primero, or what Rolex terms the Cal. 4030. Why is this an advantage over the 116520 / 116500LN? Again, it has to do with connecting the past with the present. We are slowly approaching a time in the world of watch collecting when having an outside movement may sometimes increase collector appeal. Like the Pateks with the Lemania base, or the the Paul Newman with the Valjoux movement, the 16519 (and 16520) has gradually entered the WE'LL NEVER SEE THAT AGAIN dominion so cherished by collectors. This brings me to a final thought and a recent quote by Aurel Bacs: "Vintage watches bring us back to times that we miss...times when things maybe weren't perfect...but done with love."