I think this really depends on what you want your collection for. Below are several options
1. Well rounded to have examples of WWI & II firearms
2. To have examples of the big iconic firearms (Win 94, 1911, Win 70, Rem 700, etc)
3. To have examples that let you plink, defend yourself, hunt.
4. Cowboy era firearms
If I had to do it again, the collection would likely be very different. Pistols would be based on self-defense with caliber consolidation, rifles based on what I like (lever actions) and caliber consolidation, shotguns on self defense & clay sport shooting.
My big thing is to just minimize the cartridges I need to keep. I'd probably come up with something like below ( don't have many of them now). Also, I'd get more than one so that if one breaks, is stolen, lost, etc I'm not without. I'm also a big fan of buying spare parts for things that are likely to break.
Pistols
Sig 220 x 4 in 45 ACP (two is one, one is none)
XD carry size in 45 ACP x 2
Glock 27 x 2 in 40 cal (grrr, different caliber, it woudl really depend if I could carry in CA. If not, I wouldn't purchase them; I'd also give serious consideration to their subcompact 45 ACP or likely consider an XD in 45 in subcompact).
Rifles
Marlin 336 in 30-30 x 2
1894 in 45-70 x 1
now that the lever gun fix is done ...
a good Scout rifle in .308
M1A in .308
Blaser 93 in .308
Shotguns
Mossberg 590 x 2
Mossberg 500 x 2
In terms of calibers, I'm down to 45 ACP, 40 (maybe), 30-30, 45-70, .308, 12 gauge.
Yes, I skipped a 22 LR. Don't get me wrong, I love them and have shot them a lot. These days, my biggest limiting factor is time. As such, when I shoot, I'm not too concerned about ammo costs.