Yup, quite an "open" question you have posted.
But here in the US it would be have to be an AR rifle in 5.56 and a 1911 in .45ACP.
And even though you didn't ask, a shotgun in 12ga, perhaps a Mossy 500?
Shotguns can adapt to the mission with different length barrels and stock configurations--28" hunting barrel with full-size stock, black-powder barrel, rifled barrel with slugs and cut-down cylinder bore barrel with pistol grip (for concealment). As far as ammo, mission-specific rounds can be constructed (ie: flechette rounds) because "legality" of such rounds will no longer be an issue (same with short-barrel anything).
When the SHTF, ammo WILL be a currency. ALL the military calibers will have value: 9mm Kurz (.380 ACP), 9mm, 5.56 (.223), 7.63x39, 7.62x51 (.308), 30.06 and .45 ACP. Shotgun rounds will also be valuable, specifically 12ga, 20ga and .410". And of course "law enforcement" calibers such as .38 Spl, .357 Mag and .40 S&W. Also valuable will be "specialty" hunting/sniper calibers such as .338 Lapua, .270, .300 Mag, 10mm, .44 Spl, .44 Mag. and .50 BMG. And you can't discount the value of the smaller calibers such as .22 LR (duh!
), .25 ACP, .32 ACP, .17 HMR, .22 Mag, 5.7 and also air rifles in any caliber. I probably missed many calibers but I think you get the idea. In a SFTF scenario, a box of 30-30 will NOT be worth more to me than a box of .223 (for example).
Reloading components and explosives will have great value. Lead may even become more valuable than gold! Could you even cast a decent bullet out of gold or alloy thereof?