The largest gun safe/vault I have ever seen...
There is a gun shop located about half an hour away from where I live. It has the largest safe/fault I have ever seen. The gun shop is in an old bank building and the offices are used for various things, one has a few normal gun safes and some camo and safe accessories on display, another has other firearm accessories and some antique ammo on display, another has blackpowder shooting items and regular ammo from floor to almost the ceiling. The main foyer/old teller area has his wife's jewlery business and she has one office with train sets taking up one entire wall.
However, the jewel of the building is the old bank vault. On the inside of the vault are about 80-90 firearms around the interior of the vault, with a display case in the middle with handguns and scopes. I can just imagine how many guns you could put in there if you just set up gun racks like library shelves. I could see maybe at the least 300 gun capacity if not more if you build the racks correctly.
I guess that the second best vault/safe would be to buy a nice gunsafe door with frame and devote one entire interior room to firearms.
As for normal gun safes, I view them as more than just secure firearms storage. With the extra space available you can store important papers, jewelery, cameras, laptop computers, family heirlooms and other items of value. That way you don't need a bank safe deposit box. You can also store some ammunition with your firearms if you wish.
Just because you have a big gun safe doesn't mean it has to be filled with firearms only. There are lots of things that can be stored for safe keeping in them.