This is an interesting and more complex question that it appears .
For several months, I have started concentrating on finding a some "grail" (dislike this use of that word!) guns in lieu of my usual M.O. of lesser condition "rescues". The rescues will always have a dear place in my heart and ...no I'm not at the point I will not bring an orphan into the fold. However, several guns have been on my want list for years and I find myself focusing on those of late. The occupants of this roster tend to be more rare (read that EXPENSIVE!) than my run of the mill acquisitions. My local pawn shop haunts aren't likely to produce any of them.
After a multi year search, I finally got a 1st Gen Colt SAA .38-40 with a 4 3/4" bbl. that hadn't been refinished into oblivion I could actually afford without taking out a 2nd mortgage.
Some others have found their way here like a Benelli B76, Colt 1878 DA .45 from the Philippine Constabulary, Colt 1878 .44-40 DA from Atlanta PD, and most recently a S&W Model of 1926 3rd Model from Providence RI PD. etc.
Through my years of collecting, I've successfully acquired a service revolver that was carried on duty in every state and from many other eclectic agencies.
Circling back around to the original question, my "want" list is going to sound more "high end" than I ordinarily seek out.
A S&W American from the Nashville Police would rank way up there in the rare air along with a San Antonio PD Colt SAA 1st Gen .45 from 1927 when they issued them. A 4" Model 11 would be awesome too!
Realistically, a Triple Lock .44 & a Registered Magnum would be nice. A 4" K-32 or M&P and any revolver issued by the FBI would fit nicely as well.
Best,
Charles