I would pick whatever gun was most appropriate for ridding the earth of the f$#@@ck nut who decided to pass the law limiting us to only one gun!!!
Ha, but seriously,  I'm not really sure.  I love my SKS, but it's not top of my list if money is no object.  Neither would an AK, butif money is a factor a high quality AK would probably be my choice.  
I really like my friend's AR15, but the cleaning issues make me hesitant.  I've seen his fail to feed right when clean but lacking a little oil.  .223 isnt a bad round all in all, but I like how my 7.62x39 will go right through a tree.  
I also own a winchester 94 (30-30) and a remington 870.  Both good choices in the bush rifle or shotgun catagory.  The 94 is just so sexy and smooth.  Still, if I catch a soon to be deceased burglar in my house, something semi-auto would be nice.  870s are awesome shotguns, but there's still something about the bayonet lug and slamfire capability of the winchester 97 (or norinco remake) that has a lot of apeal.  What's scarryier than a shotgun being cocked in your face?  A shotgun with a bayonet pointed at you!!!
Not sure if I really have the experience to call this one well, but I guess I'd have to say something around a .308 caliber with some kind of shotgun mounted underneath.  Fantasy?  mostly, but Im sure someone's tried it before.  
Though a M16 w/ m203 would pack a nice little punch.....