IMHO, a "classic" is something the neophyte or non-hobbiest will immediately identify or associate w/ something significant, and something the enthusiast will want to add to his/her collection because of its history or 'coolness factor'. This is not to say that something that is classic is necessarily collectible, just that it is readily recognizable and/or desirable.
With that in mind, my list of classics (and their associations) are:
AR-15: The American GI rifle
Beretta 92: The American GI pistol
Remington 700: The American sniper's rifle
Glock 17: The first plastic gun [that holds 33 armor-piercing dumb-dumb [sic] bullets]
SIG-Sauer 226: The G-man gun
AK-47: THE assault rifle [that fires armor-piercing dumb-dumb [sic] bullets]
SKS: THE truck gun
Desert Eagle: The BIG gun