Um... Colt didn't make the Garand.
You can ask 100 knowledgeable people this question and get 100 good, but different, answers.
I'm not a shotgun guy so I can't speak to that issue. In my opinion the 5 most significant handguns were...
1. The Walker Colt. Not the first revolver, but the first legitimate fighting handgun.
2. The 1873 Colt SAA. Ushered in the concept of a cartridge revolver in a serious caliber.
3. The Colt 1911. The most successful and arguably best semi-auto pistol design.
4. The S&W Military & Police Model. The grandfather for all DA revolvers.
5. The Ruger Standard Model 22 pistol. The gun that built Ruger and was the first affordable sporting pistol.
The 5 most significant rifles...
1. The Brown Bess musket. Ruled an empire.
2. The 1860 Henry. First in a long line of lever rifles. Some would argue for the Spencer, but I think the Henry outshines the Spencer for the most part and it becomes the ancestor of repeating rifles.
3. The Winchester Model 94. Essentially a variant of the Henry, but its incredible success has to rank it on its own merits. How many of these have been made?
3. The Mauser Model 98. The best military rifle of its time, it became the basis for all subsequent bolt action designs. Nobody has really improved on it.
4. The AK47. Sure, the Krauts did it first with their STG44 but the AK is the one we all think about when the term "assault rifle" is mentioned. Can 30 million Communist insurgents be wrong?
5. The Ruger 10/22. The rifle version of the Standard Model pistol.
that's my list. I'm sure 99 others will follow. BTW- I own 3 of the 5 rifles and 4 of the 5 handguns for a 70% average. Need to pick that up a bit.