Crowbars are probably the most versatile entry tool. They can bust through just about any lock, chain, door, or zombie eye socket. If you can only choose one entry tool, this would probably have to be it. If you are carrying the crowbar in your BOB, you may want to consider a titanium model.
Breaking a Pad-Lock
To break a pad-lock place the straight end of the crowbar between the lock body and the u-shaped loop, then apply as much force a possible to the other end of the crowbar, separating the lock from the loop.
To open a locked or barred door with a crowbar, place the straight end of the crowbar in the gap between the door and frame, as close to the bolt as possible. Apply force away from the center of the door until the bolt comes free of the doorframe.
Alternatively, if you are on the inside of the door (the door swings towards you when open) you can use the flattened ends of the crowbar to remove the hinge-pins.
Dispatching a Zombie
A crowbar can be used used to dispatch a zombie in two different ways, bludgeoning or puncturing the brain through the eye socket. A crowbar should only be used as a weapon if it is sufficiently long to maintain a reasonable space between yourself and a zombie.