I've always thought.. it's nice to have a rimfire, a handgun, a shotgun and a rifle. With a blackpowder to boot.
Then I think....For the rimfire... Well you need to have a 22lr in a bolt action, (Remington 512 for me). Also, you need a revolver chambered in one (Single Six for me). Then you can't leave out a Marlin 60 for the autoloader (some may say Ruger 10/22), plus a lever action Henry and a pump action Model 12. And an autoloading handgun by way of Ruger MK.
For the centerfire handgun, autoloader I like a Glock to carry and not worry about scratches, getting uglier etc but the 1911 is my all time favorite, in all sections of firearm. Revolver, 686 S&W tops my list with the Super Blackhawk being my favorite magnum.
For a shotgun, I say all you need is a pump action 870 Wingmaster. Then, my favorite shotgun is the Browning Light-Twelve, so you need that to cover the autoloaders. But what about the double barrel. For that, I like the American SxS's, a Fox Model B or if you have the coin, a Winchester 21. O/U, maybe there's your chance to buy across the pond and cover your "foriegn" section. Single shots shouldn't be forgotten either, my first gun was a 410 H&R break action. An old Mossberg 183 to cover the bolt action.
Rifle wise gets difficult for me so I'll be simple. Have to have a lever action, 30-30 is ideal but 45-70 from an 1886 is my lever choice. Bolt action is hard, tie between a pre 64 model 70, and the action that birthed it, a Mauser 98k. Break action rifles, I have no skin in the game along with pumps.
Blackpowder, I had to have a "Hawken style" .50 cal rifle after seeing Jeremiah Johnson as a kid. Along with an 1847 Walker after seeing, yes, Josey Wales. But then I needed an 1860 Army, just because.
As you can see, I've convinced myself of many reasons for firearms purchases. I'm still missing many.
Edit: I forgot the military section. Many reasons or ideas of what a collection could or should be, it's all in ones head.