.45 Colt
Reasons:
History.
145 years of experience.
Accuracy
Can be loaded with BP or smokeless.
Availability
Can be hand loaded for a wide range of uses.
You can get a rifle chambered in .45 Colt. And if the revolver is a Ruger you can use the hot loads listed in the reloading manuals for both.
Not as popular as those listed BUT I'd have to select a 38-40 (38WCF) because I have an old original Colt Lightening in 38 WCF. Have to have pistol and rifle match ammo, no?
While I would say 44-40 because that is the first thing that comes to mine when I think of an old SA Colt I would have to answer 32 WCF because I have an old Winchester of the same era already and to me it makes sense to carry one cartridge for both.
Since the OP specified * Antique SAA * , I will presume he meant what he said, and is speaking of pre-1899 Colts .
.45 Colt is excellent ctg, most popular in numbers from that era, but is in highest demand in modern times.
I personally have strong feelings for .32WCF.
.44WCF had inconsistant chambers ( & significant difference between Colt and Winchester chambers).
Leaving us with .38WCF . Good ballistics, and better chamber consistency.
But alas, I have missed my window of opportunity . Twenty-ish yr ago I potentially could have prioritized my self to quasi-afford a black powder frame SAA . But even re- cylindered and re-barlelled 1st Gens are now beyond me .
.45 Colt, for the simple reason I own modern revolvers chambered in it. That said, be sure to stick with black powder pressures if you plan to shoot it.