A key only deadbolt. You have to look around a little. I think in recent years they have become more difficult to find because they are afriad people locking them and not being able to quickly unlock them to get out in case of fire. It's only used when I'm gone, normally for extended periods. But anyway a deadbolt with a key on both sides to lock/unlcok and not the little handle you turn to lock it from inside.
I bought mine from a company that does security installations suggested by my sister. It has a longer bolt to it so that it goes thru the jamb and angle steel frame behind the jamb.
There are lots of other options. A good friend of mine put a larger safe than would fit thru the door and built the house around it. Gun safes that large aren't terribly expensive, Less than $5k and that's a very large gun safe. You can buy doors that are basically the same as a gun safe door that you can put on a closet or build in a safe if you are building a new house etc, but that does't protect anything you put inside from a fire like a regular gun safe.
To me the best and cheapest option was to reinforce the door jamb.