What is the best zombie-defense weapon? Actually, the answer really varies. You may say that the best weapon hands down is the super-badass sniper rifle made in Russia with the custom scope handcrafted by children in Finland. If you don't have access to that rifle, that's a worthless rifle to you. The best zombie weapon for you is the weapon, A, you have access to, and B, that you know how to use.
If we are just playing fantasy, I would say that the best zombie-defense weapon is a nuclear bomb. How about a magic wand that when you zombies they turn into unicorns? In the land of reality, you only have access to the weapons that you would actually have. Remember, fighting zombies is not a video game; you won't be automatically an expert shot and you will not know how to automatically load every weapon, use every weapon, and find ammunition. You really need to stay within your own abilities and be realistic.
If you have never loaded a firearm, if you have never cleaned a firearm, if you have never used an advanced combat blade like a katana, probably, they should not be your first choice. You need to have training, you need to know what you are doing, and you need to stay within your abilities.
The best zombie weapon for you is the weapon that you currently possess. Unfortunately in a catastrophic zombie outbreak, you may not even have time to go home. You may not have time to go to the gun store or to go to your dad's house. Possibly, the best zombie weapon for you is whatever is sitting right next to you right now.