FerFal wrote a post on first guns. To be honest I sort of hate this kind of hypothetical stuff but somehow keep coming back to it. Everybody should get at least a good centerfire pistol, a pump shotgun, a centerfire rifle and some sort of .22. I suggest a Glock, a Remington 870, an AR and a Ruger 10/22. Having bought them I am fully aware these are not cheap guns. Matter of factly I believe almost all people (its tough if you are truly disabled and on a small pension, etc) would be well advised to get quality modern firearms like the ones I named above. Scrimping, saving and working extra hours to get good guns sucks (I once worked full time over Christmas Break to get an AR) but you will not regret it in the long run. Guns aren't the place to scrimp and try to save a few bucks. Commander Zero wrote a great post on this awhile back.
To not get too focused on debating this gun vs that gun really the possible combination are almost endless and provided you stick to common caliber (.38/.357mag, 9mm, .45acp for pistols and .223, 7.62x39, 30-30, .308, 30-06 for rifles of course 20 and 12 gauge for shotguns) firearms in common model/manufacturers (S&W, Ruger, H&K, Glock, Sig, Colt, Browning, Winchester, Mossberg, Remington, Marlin, AR, Mini-14, AK, FN-FAL, HK-91, M1A, etc all) you will probably do fine.
You need a centerfire pistol for carry and generally being discretely armed. Shotguns are probably the king of home defense and generally a great utilitarian gun to have (we will talk more about them later). Rifles are useful for hunting, plinking, and really the only decent option to shoot just about anything past about 100 meters. I think everyone should also have a .22 (I would suggest it be a rifle) because they are so darn useful and dirt cheap to shoot. These 4 guns will not deal with every possible situation in hunting, plinking and potential defense. The rifle you want if things to truly to hell is almost invariably a magazine fed semi automatic and not the one you would want to hunt Moose with and it might be convenient to get a subcompact carry pistol for EDC. I suggest these 4 basic guns because they will be good for most situations in defense, plinking and sport and are the foundation of any good home firearms battery.
However what would be best for people to get while they are on their way to this basic 4 is part of the question at hand.
I used to say a shotgun but FerFal's wisdom changed my mind. Some folks say a rifle but I think they are more wrong than the shotgun crowd. The thinking behind a pistol is that it is the only weapon you can have with you all the time without being arrested or causing a scene. Also you can answer the door or investigate that strange sound outside armed without giving Granny Smith your neighbor a heat attack. A shotgun is probably better strictly for home defense and a rifle might be better for a genuine TEOTWAWKI but both fail to take into account far more realistic and probable situations.
Shotguns for home defense also came up in the post which inspired this one. I am not saying that a pump shotgun is a talisman. No matter what any add says or infers no gun is going to turn you into a Grizzled Master Sergeant from CAG, sorry. I honestly believe the best all around gun for home defense is a short barreled pump shotgun. Mossberg and Remington are the best options since Winchester kicked the bucket. Some have noted that shotguns hold a relatively small amount of ammunition @5-9 shells and used that for a reason either pistols or rifles are better for home defense. I would mention that the average gunfight is a couple rounds at spitting distance but well we plan for the worst not the best or average. Shotguns are better for home defense than pistols for many reasons. First without getting really far into the weeds on ballistics and round selection I don't think anyone has ever seriously questioned the man stopping power of 12 (or for that matter 20) gauge shotguns loaded with buckshot. Someone who takes a load of buckshot to the torso is probably going to be out of action very quickly or at lest as quickly as any other weapon we are discussing.
The other fact which I think mitigates the relatively low capacity of shotguns is that most people shoot them far better than pistols. No matter what anyone says you have to aim shotguns to hit anything. The reason I say people shoot them better is that the combination of being on your shoulder and a good cheek to stock makes shooting much more intuitive and easy. Take a bunch of folks to the range and have them shoot at a variety of targets at defensive ranges with a pistol and a shotgun, bet almost all of them will do better with a shotgun.
This has turned into a really long post and I am just getting to the actual new unique thought. The just get a pistol and a rifle idea (or getting them first in priority) has popped up here and there from time to time. To be honest I can only see two situations where it makes sense. The first is if you want/ need a rifle and space is really limited (living on a boat, etc). The second is if you had a rifle already, bought a pistol and are waiting till you have the cash for a shotgun. Lastly maybe for whatever reason you just really don't want to have a shotgun, can't see why you wouldn't want a dozen of them but different strokes for different folks.
The reason most people don't just decide they need some guns and go to the store to pick up a Glock, a Remington 870, an AR, and a Ruger 10/22 is that they cannot afford to do so. Guns, particularly modern quality guns are expensive. Really the only reason this comes up at all is that it takes lots of folks some time to save and scrimp together enough cash to get a nice basic firearms battery.
The reason I believe a rifle as a second gun (after a pistol) is a bad course of action is not so much about the individual attributes of shotguns vs rifles but is all about economics. You can buy good used Mossberg or (to a slightly lesser degree) Remington pump shotguns for less than $175 all day long until you run out of cash or get bored. These days you can't touch an AK for much less than $450ish and once you get 20 mags at about $9 a piece the cost goes up considerably. An AR which wasn't made in somebodies basement is going to cost at least $700 or more depending on how name centric you are. Mags cost about $12 for Mil Spec and a bit more for MagPul. I don't even want to talk about how much good semi auto .308's cost, the M1A I want and will some day get is going to cost about a pound of flesh.
For a shotgun all the accessories you need are a sling, a buttstock shotshell carrier. Plus of course ammo. You can get a good shotgun with its needed accessories for less than you will spend on rifle mags alone! Shotguns are far cheaper than quality defensive rifles and thus you can have a long gun and a pistol far sooner by going the shotgun route. This way you will be as well armed as possible while saving up enough coin to get a rifle.
Here is where someone is going to mention surplus bolt guns. I covered my opinions above but lets reiterate as it applies to the long gun for defense. Yes some older style and WWI-II surplus rifles can be purchased for much less than modern quality rifles such AK's and AR's or various .308's but they are almost universally a horrible choice for defense particularly inside the home or in close quarters. They are big and heavy/ cumbersome, slow to reload and a potentially deadly choice given the potential for multiple adversaries in close quarters. For someone married to the idea of a close quarters defensive rifle without the desire to get something semi automatic (and probably mag fed) I would suggest a lever action 30-30 as they are relatively cheap, compact, fast to reload and generally the best of the undesirable options in this range.
In conclusion I think it is smart to buy a defensive pistol, pump shotgun, centerfire rifle and a .22. I believe quality modern guns are worth the money and that most people should make the choices necessary to get them. For those who have no guns buying a defensive pistol first and a pump shotgun second is IMO the best course of action with a centerfire rifle and a .22 to follow at a later date when funds allow.