Lots of really great choices in this thread.  Not just in this thread, 12-gauge shotguns seem to be the world's favorite gun.
Ironically, tonight the firing range just got in a new Auto-Ordnance M1 .30- cal. carbine, my fond memory that got this thread started.    
Oh wow wow wow wow!  That sucker feels as great as the first time I ever cradled it in the Air Force.
What a perfect gun.  
-- that clanky sound of the bolt, 
-- the looseness yet aura of utter dependability, 
-- the beautiful o-ring back sight with the unique front sight of blade flanked by outwardly bending guards,
-- the confidence building magazines,
-- the attractive just-right-size bullets,  
-- the serious sling arrangement, 
-- even a bayonet holder (I know, don't stick one on in most locales), 
-- the absolute ideal weight, length, balance, and overall feel for me.  
You never forget your first love (Mom was talking about mates, but it applies to weapons too).  
I don't really need one now.  At around $700 in walnut it's too expensive just to have one's lifelong absolute favorite gun.  
It would be an indulgence.  
Totally unjustifiable.
But ... if I could have only ... ONE gun ...