Colt for changing manufacturing processes and making the first practical revolver.
Mauser for the bolt action that's still copied today.
Maxim for the machine gun and silencer.
Browning for, well, it's already been said.
The rest blend together for me.
I voted John M. Browning ..... YOU did NOT list Smith, or Wesson or B. Tyler Henry or Oliver Winchester ..... or "Carbine" Williams ...
THIS is my 3000 th post .....
John Moses Browning designed de novo and improved virtually every aspect of gun engineering and manufacture. We still, to this today, utilize many of his designed unchanged and unmodified.
The man is the Nikola Tesla of the firearms world, and that is the highest praise it occurs to me that I could give anyone in his respective field.
For design, innovation, and the timelessness of his creations (as another poster pointed out, many are virtually unchanged), then it's John M. Browning, hands down.
2nd place for me would go to Mr. Kalashnikov; imagine what he could have done in a western society?
Like others have stated, Browning has designed so many firearms that have withstood the test of time and are ageless. The 1911 has to be the most copied gun in the world.
No Sh**. The Mauser brothers had something going on.. and yes, there are many men that brought an epiphany to production (Garand and Stoner for instance). But, J. Moses Browning is your man.
Wow. That's a tough choice when you first look at the list.
Process of elimination... Garand, Glock, Gatling, Maxim and Thompson are off the list in the first round... easy... one hit wonders, if you will.
Since the question is "largest contribution to the firearms world"...
Even though I think they are by far the most popular (AK's and AR's) you have to ask, what are the other accomplishments? Stoner and Kalashnakov are off the list.
Mauser actions are still used today and they are the basis for arguably the best bolt rifles ever made, but aside from that? The Mausers are off the list.
That leaves Sam Colt and JM Browning. As posed in the OP "pick the one you think made the biggest impact in firearms design"... since many of Colt's guns (and Winchesters, and...) were actually Browning designs, the nod goes to...
Browning has to be #1 he gave us the first lever action shotgun, first pump, first autoloading. The .50 cal machine gun and one of finest pistols ever made.
Look at it like this; take away the #1 design from each one of them and who's left? Browning, Colt and the Mauser boys. The rest were one hit wonders(yes, all Glock models count as one).
Take away Colt and Mausers as the were pretty much one trick ponies, 6 shooters and a bolt actions(yea, yea that broomhandle was sweet).