The Merwin and Hulbert would be a close second too.
Merwin & Hulbert revolvers are a marvel of 19th century engineering an ingenuity. I have presented photographs of one to aid in this discussion, a Pocket Army Second Model. You will note the tolerances and the tight fit of all components; some say that the revolver contains such strict tolerances that suction will pull it closed on its own, or nearly so. I own several Merwin & Hulbert revolvers, most with condition, and I've never experienced anything like that. Many have lauded the Merwin & Hulbert, and its popularity increased significantly after Art Phelps wrote a book about them, praising them profusely.
It's hard to say whether or not they are better than Colt Single Action Army revolvers, but, without a doubt, the fitting is far superb to that of a Colt Single Action Army revolver. It definitely required much individual hand fitting during production, which would make the cost to produce today prohibitive.
I would suggest that longevity is not necessarily a decent criteria to determine if a revolver, or any 19th Century firearm, for that matter, is superior to another. A firm can produce a great firearm, but be managed poorly, leading to its demise. Merwin & Hulbert produced a top notch revolver during its 15 year existence, 1876 to 1891. In 1891, the company filed for bankruptcy. It wasn't that the quality of the product was poor, just that management was.
Other considerations with regards to what was the best 19th Century firearm. Number produced? Others have mentioned the Winchester Model 1894. Millions were produced, any post 1964, according to most, are slightly better than junk. In my opinion, anything after mid to late 1920's suffers in quality. In my opinion, even though only 159,994 Winchester Model 1886's were produced, they are far superior.
Obviously, a fair number of Colt First Generation Single Action Army revolvers were produced (over 357,000), but a large number were produced for use by the military (Cavalry and Artillery Models). Perhaps familiarity with these as a military firearm led to their popularity with private individuals later in life. Also, marketing and the size of a firm would have something to do with production numbers and capacity. No doubt about it, a Colt Single Action Army revolver, first generation, is a fine firearm indeed, but was it the best one ever? That has no definitive answer.
One last thing to think about. I would imagine that if one intended to do a lot of firing from horseback, the Smith & Wesson New Model No 3 revolver, a competitor at the time, would be superior to a Colt Single Action Army revolver, due to ease of loading and unloading. Crack open the (top break) revolver, insert six rounds easily and quickly, fire said six rounds, crack open the revolver, quickly dispense 6 spent rounds on the ground, and repeat. Loading and unloading a Colt Single Action Army revolver requires much more manual dexterity.