There are several door lock options, but to be as safe as possible, one must also consider the material of which the door and door frame are made and the location of any nearby windows.
Keep in mind that the door lock can be stronger than the door. If the door is thin, flimsy pine, an expensive lock may prevent a burglar from turning the knob but will not prevent him from simply kicking in the door. Also be aware of any windows that could also serve as entry to your house. They should be locked and, if necessary, barred in such a way that allows for easy exit in case of a fire but prevents entry. Windows near the door should be small enough and far enough that a burglar could neither climb through or reach through to unlock the door.
Chain locks have the advantage that you can look through the cracked door to see who is there and converse without completely unlocking the door. Additionally, you must close the door to unlatch the chain from its runner - no one can reach inside and unlatch it. However, a peephole and an intercom will give you the same benefit of communication without the risk of opening the door to rely on a single lock. The metal chain itself may be very strong, but any chain is only as strong as its weakest link - and here the weakest link may be the wooden doorjamb the lock is attached to.
Some companies make door jams that form the long side of a triangle between the floor and the door. Supposedly the friction between the rubber foot against the floor, coupled with the rubber grip on the door handle, makes the door impossible to open from the other side. This sounds possible in theory, but this author has tried a few of these devices and never found one that worked as claimed.
A lock in the doorknob can be easily picked within minutes by a professional with the proper tools or even a screwdriver. The lock works through pins that need to be lifted in a certain order and to a specific height; this can be achieved with lock picking tools. Also a credit card can be used to open the door, or a crowbar can be used to warp the door enough so the lock doesn't matter. This kind of lock is not secure.
A bar across door, as you see on many glass porches, can be effective, assuming the door is not make of glass. This is the most extreme option, so if you choose this, be very deliberate about installation. Weak points include the type of material, whether wood or metal, used to bar the door; the joints used to hold the bar to the wall on both sides; and the strength of the door itself, which could in theory break around the bar.
Consider wrought iron grill screen doors in addition to standard doors. They look pleasant but are difficult to get though when locked. This will satisfy the homeowner looking to be secure and the homeowner who wants a pleasant looking home. Also, they let in the cool breeze but keep the flies out. And that's what security's about - letting in things you want in and keeping out those you want out.
