wondering what the holy grail lep of AZ is?

It depends on who you talk to. If you talk to a giant skipper collector, he would undoubtedly say Megathymus ursus. A Saturniid collector would probably say either Rothschildia cincta, Automeris randa, or Sphingicampa raspa. A Sphingid collector would probably say Dolbogene hartwegii or Proserpinus vega. An Arctiid collector would say Dysschema howardi. Collectors now have most of those species pretty well dialed in. A few of them like M. ursus though are not common anywhere at any time.

A female Dysschema howardi is by far the prettiest/most impressive Lep in AZ.

I've caught literally hundreds of P. multicaudata, and they seem to have pretty uniform size of about 4.5" wingspan. I think I may have taken a female that was 5.5" wingspan about 7 years ago. The only leps in AZ that I've seen with 6+" wingspans are A. oculea, H. c. gloveri, and Ascalapha odorata.

Personally I think a fresh female Speyeria nokomis nitocris is prettier than a female P. multicaudata. If you include moths though, I'd have to say that the female Dysschema howardi, Hemileuca electra clio, or Sphingicampa raspa have the best colors. If you include small moths, I'd have to say one of the Stiriines, probably either Chalcopasta howardi or C. territans, both of which are a bright metallic gold.

As far as trophies go, it would probably be one of the stray Sphingids like Erinnyis crameri, E. alope, Eumorpha satellitia, E. vitis, or Isognathus rimosa. All of these turn up every once and a while and may even be temporarily residential. I've never heard of anyone finding larvae of any though. They are never predictable, and the only stray that's really even somewhat routine is Manduca occulta. It's all very subjective though.

the holy grail is usually the one species that you have longed to have for years but don't have it yet.

For a complete overview of AZ moths, google Bruce Walsh's "Moths of Southeastern Arizona" website. I agree that a female Dysschema howardi is the most spectacular moth, while P. multicaudata is the most spectacular butterfly (along with P. pilumnus, a rare stray). I wonder how the sizes claimed for multicaudata were measured.

I have to agree with Evan that it really depends on what you are after. Arizona has so much to offer that it's really hard to pin down what "the holy grail bug" would be. The first trip for me was in late August, mainly for Dynastes granti. I collected so much more than just beetles on that trip, that I quickly realized I needed to expand my efforts. Last year I went in mid July, mainly for moths, and I did manage to get D. howardii, S. raspa, and by a stroke of luck (literally), I net captured a beautiful female P. multicaudata while I was following Evan down a gravel backroad to check for Witchmoths under a bridge. What fascinated me was the fact that each location held something different. One place would yeild lots of Citheronia splendens, and another we found lots of Sphingicampa raspa for example.
So if you have a target species in mind you need to narrow your search down to known locations for the species, but never hesitate to collect something you might consider interesting, because you never know when the opportunity will arise again. You might find out that the occasional "rare" bug you happened upon is actually more rare, and more prized by collectors than what you were after initially. I have learned a long time ago not to fixate on just one species, especially when making a distant trip. I collect as much as I can, and sort through the material when I get home. I make every effort to mount and label every specimen I collected. Sometimes I am pleasantly surprized that I got a "rare" bug I didn't know about at the time of capture. I think the term "holy grail" can shift from one bug to another as time goes on in any collectors career.