IMO, it's kind of a toss-up between the .45 Colt as originally loaded with 38 gr. of powder under a 250 gr. bullet and the .44/40 (.44 WCF) with a 200 gr. slug over 40 gr. IIRC, these can't be easily duplicated with modern solid-head brass, as the case just won't hold that much powder.
Mike Venturino, who's probably done more systematic testing with BP cartridges, revolver and rifle, than anyone else in the game did a very interesting article on the subject titled "Old West Pistol Power" in the Jan/Feb issue of "American Handgunner".
While the main focus was comparison of relative penetration as tested with a baffle box, the chronograph numbers should shed some light.
.44/40: 205 gr. LFP, 40 gr. Swiss FFFg: 1060 f/s from a 7.5" SAA
.45 Colt: 250 gr. LFP, 38 gr. Swiss: 1050 f/s also from a 7.5" SAA
BTW, testing was pretty comprehensive. Included a .31 Baby Dragoon, .36 1851, .44 1860, Richards conversions in .38 Colt and .44 Colt, .44 S&W American from an original #3, .44 Russian from a Navy Arms replica #3, and .45 S&W from a SAA in addition to the above. He compared them with modern .32 ACP, .40 S&W, and .45 ACP to add contrast.