Personal
From my own experience I can say that the rifle, hands down, was easiest to achieve good results in the least time.
I've used a bow (old fashioned Pearson 30# fiberglass recurve), rifles of various sorts, and a pistol or six.
With the least training and the least experience, I have consistently gotten the best results with a rifle. Some better than others.
At fifty yards I'm better more consistently with a pistol, given the same amount of practice as for a bow.
I got reasonably good with the bow inside of fifty yards, but consistency kinda stank.
I have better control with a pistol than a bow, and this was achieved with fewer range hours.
I would say that consistent good results with a bow requires more skill and more discipline, especially at ranges beyond fifty yards.
I remember during rifle qualification in the USAF hearing that some of the guys had scores of fifty and less out of a total possible sixty valid hits. Wait -- you mean someone was able to miss that target with a rifle? I was arguably one of the greenest guys with a firearm in my squadron, and all I did was line up the sights, pull the trigger, do it again. Sixty for sixty. And I figure if can do that, anyone can do that.
If a green recruit with only one day's practice with a rifle can keep all sixty on the silhouette, that has to be my choice for "easiest to master."
Pistol would be next.
Bow would be the most work for me to master. There are more issues than simple accuracy. It seems to me that strength and stamina/fatigue play a bigger role than they do with firearms.
For me: most challenging = bow.