The ultimate backup in my opinion is a combination of awaredness, training, and tools.
Awaredness being noticing the situation around you, noticing an attackers weakness and strengths in just a few moments and general alertness.
Training being well training.
And tools being the weapons, gun, knife kubaton, big stick or what have you.
If I had to pick an ultimate backup weapon, the one that its only use would be where I was about to die and it was the only way to stop an attacker, what I would pick is a dagger I have. Its a neat little home made deal, wood handles pinned on with tacks that have been gound flush, and a blade made from bandsaw blade. its about 10 inches overall with an about 7 inch blade. carries nicely in my boot.
Now if I carried a gun as my main defense weapon this choice would be different.
But since I am 15, and dont want to break the law and thus render myself unable to buy or own any guns if I get caught, I carry a knife as my main defensive weapon.
Its a knife made by Emerson, called the Commander.
http://store.knifecenter.com/pgi-Product Spec?EME3BTS
Its a production version of Ernest Emersons custom made ES1-M folding fighter, designed for SEAL team use.
This would be my choice for a backup if I carried a gun.
It opens faster than any automatic knife ever could, by the simple fact that if done right it comes open when you pull it from your pocket.
It has a little hook type thing on the top of the blade, incorperated into the thumb ramp, that catches on the edge of your pocket when pulled out that pulls the blade open.
Also the recurve style blade naturally pulls the blade deeper into whatever is being cut with it, and in defensive use this creates a massive wound channel.