Probably not in the correct order, but a few of the greats:
1. Browning (1911, 1917, 1919, M2, BAR, Winchester lever actions, lever, pump, over under, and semi shotgun, host of pocket pistols, and a slew I'm forgetting. He should be number 1-5 at least. Any one of his guns was an amazing invention, let alone dozens of them, yes we still use the M2, and the FN MAG, the most popular GPMG in the world is basically a BAR flipped over converted to a belt fed).
2. Mauser (The best and most copied rifle action ever).
3. Maxim (Father of the heavy machine gun)
4. Garand (M1 Garand)
5. Whitney (Interchangable parts for manufacturing, probably should be much higher on the list if you think about it)
6. Heckler, Koch, & Seidel (Roller locking bolt)
7. Saive (FN 49 rifle, FN-FAL, High Power)
8. Kalashnikov (though this is arguable. He didn't really invent anything new, he just recombined old ideas together well).
9. Horace Smith & Daniel Wesson
10. Sam Colt
11. Col. Lewis (His gun isn't as famous as it deserves to be)
12. Col. George Chinn (Mk.19, and one of the sharpest experts on the machine gun who has ever lived)
13. Hugo Borchardt & Georg Luger. (Borchardt invented the toggle lock, Luger just moved it around into a much nicer weapon)
14. Bill Ruger (As much as I don't really like the man, he has come up with some weapons that have really changed the gun world)
15. Stoner. (More for his pioneering work with aluminum than anything.)
16. Mondragon (kind of a quirky design, but he proved that a military semi-auto rifle could be made, and this was before the turn of the century)
17. Berdan (I believe he was the man that invented the primer cup, IIRC)
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#267. Larry Correia (CUR, CTR concept)
Someday, you just wait. I'm working on building a prototype of the CUR. (Correia Utility Rifle). Its going to be cool, I promise.