I voted AR.
most likely to be hard to get in coming years.
Easiest to customize, etc
1911's and revolvers will be available for a long time.
Shotguns will be the last to go (despite my utter and contemptable love for a good double gun, DO NOT GO TO BASS PRO SHOPS FINE GUN ROOM, IT'S VERY BAD FOR THE WALLET)

Get a REAL NICE AR
MSTN, CMMG, LMT, something.
and lots and lots of magazines.