It depends on the era! Each age had it's critical designer who did SOMETHING to rapidly advance firearm technology.
In the early 1700s, it was the man who developed flintlock vs. wheellock/matchlock, giving improved ignition.
Mid 1700s, it was the developer of rifling in the barrel. Accuracy suddenly jumped from 50 yards to 100+.
In the early 1800s, I would go with Colt. While there were lots of other revolver manufacturers - Smith & Wesson would probably be the #2 of the late 1800s, followed way back by smalltimers like Shawk and McClannahan, et. al. - Colt paved the way for westward expansion. His design gained fame and nororiety when Gen'l Zachary Taylor sent Col. Walker to New England to find Colt and make revolvers for the U.S. Mounted Rifles for the Mexican War. Loved by the men, the Colts became the weapon of choice for the Texas Rangers and spreading onward and upward from there.
Mid 1800s, I would go with Gatling. His gun - thank God! - came too late to be of cricital play in the Civil War. Could you imagine what it would have been like, casualty wise, had it been fielded at Gettysburg, Hattiesburg, or Vicksburg? The bloodshed of WWI would have happened in the 1860s - maybe worse because of the Napoleanic army tactics of the day.
Late 1800s, Henry and his repeating rifle. It, along with the revolver, opened up the West for development. Look at the difference of the Souix vs. Custer - repeating rifle vs. breech-loading single shots. Henry brought accurate, rapid fire to the individual shooter.
Early-to-mid 1900s, Browning. Period.
Mid 1900s, Kalashnikov. Not even Browning's guns are as reliable, rugged, butt-ugly yet completely functional so that a completely ignorant conscript soldier can field the weapon with full potential.
Stoner's AR-design is great, as is evidence by the M16/variants still in use 40 years later. But, IMHO, his original design had a lot of flaws that had to be worked through. It's original form was definitely below the AK. So, I can't give him the nod.
Modern age: Barret. His guns continue to amaze, from his bolt actions to his semi-autos, giving INCREDIBLE firepower to individual soldiers/citizens who may need it.
But if I had to pick one, it's Colt. Without Sam Colt opening the west, JMB would have been designing parts for railroads.
Q