Armageddon is any situation where the normal social safety net has been obliterated. Where order no longer applies, and rules are few to nonexistent. Conventional war for example may not be considered a state of armageddon because soldiers must follow orders, and ideally everything is conducted with coordination. Chaos must always reign supreme in armageddon. Examples of armageddon therefore are the following (in order of greatest and least likely, to smallest and most likely):

1) Biblical armageddon:
Everything about Christ or whatever God your believe in turns out to be true after all. Perhaps Judgment Day is upon us and the dead are walking. Literally anything goes in this scenario; monsters, ghosts and whatnot.
2) Alien invasion:
Just like in the movies (e.g. Mars Attacks). Earth is suddenly the target of a massive land invasion from millions of little green men.
3) Nuclear armageddon:
The Cold War fears become a reality. Whether it's by China, Russia, terrorists, or just an accident, we find that most of the world's major cities are destroyed, and nuclear mayhem is upon the survivors.
4) Military invasion:
A very large foreign country invades the United States. It may not seem feasible now, but even the Roman Empire was eventually sacked...
5) Global disasters:
Perhaps a comet smashes into earth blocking out the sun for years. Perhaps a new virus plagues humanity and forces survivors to live in camps or underground. Perhaps the global economic system completely crashes us back into the stone-age.
6) Local disasters:
Earthquakes, typhoons, hurricanes, asteroids, fires, floods, riots, etc. This armageddon scenario is the most transient of them all. Within time a country can restore order to the region, but some areas may experience armageddon for a period of days to weeks.

I will only consider small arms. While a tank may be considered by some to be ideal, I only consider what one can obtain in today's society. That means rifles, shotguns, handguns.

In general, the most important factors are killing power and practicability. Everything else can fall under those two categories. There certainly is no one "ideal" armageddon weapon system. The weapons may depend on what sort of armageddon one is talking about. For example, riots may make the shotgun superior, whereas an alien invasion may make a big game hunting rifle ideal. Do not be fooled into the search of one ultimate weapon. This paper will however outline some of the best all-around weapon systems. Lastly, the type of armageddon not only influences the best weapon, but the kind of person using it also influences the choice. For example, you may be the high-risk type, storming enemy tents as often as you can. A sub-machine gun may therefore be your ideal weapon. As for me, I'm looking more at long-term survival with occasional combat, preferably as far away from me as possible. Please keep this in mind when reading the paper.

The weapon system must be portable and controllable
This rules out heavy machine guns, unless you plan to become a one man army. You can expect to be moving a lot in armageddon, and you won't always have somebody driving your .50 cal machine gun around for you. Don't forget you also need the ammo.
The weapon system must be of battle proven design
This is no time to start messing around with any high tech gizmos that are unproven.
The weapon system must be durable
You must expect your weapons to outlast you. They must be made out of hard materials designed for combat.
Ammunition must be plentiful
You can't walk into your local gun shop and buy a box of 20 rounds of your favorite ammo. The ammo you have will be either what you start out with and/or ammo you steal or take from your victims.
Weapons must be built from armageddon-ready parts
This means that your weapon has to be a reasonably popular model. If you're using old Uncle Joe's garage built rifle, you'll never be able to repair it if something goes wrong. Parts for weapons that have a large pre-existing market will be relatively plentiful during armageddon. If a part breaks or wears out, you will be much more likely to be able to fix it. Also, if something happens to your weapon, it will be much more likely that you will already be an expert in the weapon you get to replace your own.
Killing power must be strong
You want to know that when you shoot somebody or something, they're going down. It is more likely you will have to hunt large animals in the long term for food then it is that you will be hunting people. No wimpy calibers allowed.

From the criteria I have set out, one "system" dominates above all others. Very generally speaking, one must have one long gun, and one short gun. The handgun is small enough so that it doesn't hinder your movement. There is no reason in the world not to be carrying a pistol if you could. Carrying two pistols would be a personal choice, but don't forget that only in movies can one shoot with both hands. If you're in a situation that requires that sort of heroics, you can consider yourself dead anyway. The long gun must be a rifle. Some may argue that the shotgun is superior, but the shotgun has too many disadvantages. An obvious one is its short range of effectiveness (under 100 yards for sure). You want to engage your enemy as far away as reasonably possible. And besides, you can always rely on your handgun for your close quarter combat. The added effectiveness of the shotgun for close quarter combat does not outweigh the loss of general combat abilities. Therefore it is concluded that the system will consists of one rifle and one handgun. Submachine guns will be discussed later.

There are countless types of rifles out there, how can you possibly begin to eliminate some of them from the potential list? Let's begin with the general requirement that "ammunition must be plentiful." This very quickly narrows the list to rifles chambered in military rounds. There are orders of magnitude more rounds of military used calibers than sporting calibers. If you're out of ammo, it will be almost impossible for you to "run into" a crate of .375 H&H for example, while running into a box of NATO rounds is much more reasonable. With that being said, the chambering of your rifle is reduced to:

7.62 NATO (.308)
5.56 NATO (.223)
7.62 x 39 mm (Russian)
7.62 x 61 mm (30-.06)

Next let's consider the general criteria that "killing power must be strong." Your caliber must be able to not only kill enemies at medium ranges (200-500 yards), but also take down animals at those distances. This further narrows down the list to 7.62 NATO and 30-.06, which are excellent for medium sized animals (deer, elk, etc.). Although not ideal, they can also potentially be used for larger sized animals (i.e. bear). The .308 is definitely not the ideal round with everything else being equal, but it is the best round from the ones that have been mass produced.

Practically speaking, one can only consider semi-automatic or automatic weapons that are magazine fed (for obvious reasons). Considering that, we can once again reduce caliber choices down to the .308. There are simply no modern semi or full auto weapons developed in 30-.06. There are the M1 and the BAR, but those will be shown inferior to the .308 weapons. We have come a long way. The rifle criteria is now: light weight, magazine-fed, shoulder fired, semi or full auto chambered in .308. Beyond this I cannot specify anymore what is best for you. You will have to choose from the following:

FN-FAL

HK G3

M14/M1A

You can feel 100% confident in yourself stepping out into the world of armageddon with one of these three rifles on your back! For those of you who are die hard Stoner M16 fans, there is another option:

AR-10A2

The problem with the AR-10 (.308) is that it is not a mainstream rifle. Its parts have never been mass produced. For example, if something happened to your magazines or if you lost them, it would be nearly impossible for you to replace them.

The Israeli Galil is also an excellent combat rifle. Unless you live in the Middle East however, parts will be hard to come by, especially for the 7.62mm version. Featured in the photo is the 5.56mm version.

IMI Galil

We will start this process as we did with rifles. Choices of caliber are very easy for pistols: 9mm Luger and .45 ACP. These are the only mass produced military rounds that one can seriously consider. You may not be a fan of either round, but face it, that's what you've got for armageddon. Given that, one cannot eliminate the choices much more since choice in handguns is very personal, and different for most people. The following represents an excellent list:

Beretta 92 (9mm)

1911 (.45)

Sig Sauer (9mm or .45)

HK USP (9mm or .45)

Browning Hi-Power (9mm)

There are other pistols which will do the job, but the proven combat effectiveness combined with readily available magazines and parts puts the pistols listed above in the first tier.

Some may argue that a shotgun is a must. The fact is though that a shotgun is only effective at most up to 100 yards. When entering a chaotic and completely unknown world of combat, one needs a rifle. Others will argue in favor of 5.56mm over 7.62mm. The 5.56 is adequate against human targets up to 300 yards. It fails miserably in hunting, and is completely inferior to the 7.62mm past 300 yards. With 7.62mm, you will be able to shoot your enemies long before you might be in their range.

Others may argue for the use of a full sized submachine gun. This is actually not a bad plan, and weapons such the H&K MP-5 or the IMI Uzi are ideal candidates. Because they are chambered in 9mm, you could use a 9mm pistol and then only have to carry or worry about one type of ammo for both your short and "long" guns. This is a great advantage. The problem however is that 9mm is an inadequate battle round, and will not stand up to challenge during armageddon. Either a shotgun or a submachine gun would be great to have, but are not the go-to overall ideal weapon choices.

Another honorable mention goes of course to the AK family of rifles as an alternative.

It is also important to carry a durable fighting knife. Many will work, but it is important you don't buy a piece of crap that will break. Most military knives will do, although I recommend a good SOG, Strider, or Chris Reeve. The knife will hopefully not be used in combat (lest you be in big trouble!), but rather as an excellent tool. Your knife should never fail. It will be your last piece of defense. It has no moving parts and must never let you down. As we learned in Conan, "steel you can trust." Don't get fancy with plastics and ceramics. Always carry a backup folder knife and a leatherman type tool.