I like top 5 lists, so as of right now today, these 5 would be my choices. Most of these are more "collector" than "shooter" in terms of price, but what is the point of owning firearms you don't shoot?
1. Holland and Holland double rifle in 470 Nitro Express with automatic ejectors, regulated at 50 yards with a 500 grain bullet.
2. Mauser 1918 T-Gewehr
3. Anzio Iron Works 20mm Vulcan rifle. This is only because getting a Lahti is problematic.
4. 45 ACP Luger prototype.
5. Lemat Revolver