Depends on how much you're willing to spend.
Be careful, because many of the well known brand names out there mostly make boxes that aren't considered a true safe according to Underwriter's Laboratory. And the ones that are truly considered safes are usually quite overpriced compared to some lesser known names that make better products. In order to be considered a safe, a box needs to achieve at least a TL-15 security rating by Underwriter's Laboratory.
American Security makes one gun safe that is considered a true safe according to UL testing. It is the
RF6528. Compare it's specifications with the other typical name brand safe's ratings...hmmmm...
If you want to look at a really nice true UL approved safe another one is
Graffunder Safes. You'll pay a pretty penny, but they will blow away probably anything you've considered.
Personally, I can't justify spending that much on a safe right now, so I'm seriously considering the Amsec
BF6030. For the money it will still beat out a lot of the safes that are typical in the big box stores.
I suggest if you're really serious about getting a safe that you go to the
UL website and do some research on safe testing. Then compare that with what some of the different manufacturers do. Many of them don't even have their safes UL tested. They are factory tested. Factory testing is not an unbiased test IMHO. Personally, I would not trust a very valuable gun collection and other valuables to something that wasn't UL approved.
Also, if you have any insurance on your collection make sure you check out the fine print of your policy. Some of them require a UL approved TL-15 safe or better.