I'm not sure what you're preparing for, but for emergencies where you may have to defend yourself/family against other humans, I'd look at the following:

Handguns: 9mm, 40S&W, 45acp for service size pistol; 9mm or 38 Spl (the latter in older Smith J-frame for BUG use (BUG=BackUp Gun) I'd stay away from 357 Sig since it gives identical wound ballistics as 9mm, but with more wear and tear on the host piece and the shooter and is harder to come by.

Rifles: 223/5.56, 6.8 SPC, 30 Carbine, 308. This would be in bolt or semi-auto rifles. The 6.8 is clearly superior to the 223/5.56 in virtually any test you wish to conduct, and fires from the same platforms. Not as popular, but if you stock up on ammo it's not a problem.

Shotguns: 12 gauge. It's universal and tactical buckshot loads give better "performance" against living tissue than magnum loads, even though it was originally designed for small statured cops to use. The reduced velocity prevents pellet deformation and therefore gives both better patterns and increased penetration. Standard Foster-type slugs are good, and there are low recoil loads available. Where more power is needed (big animals or barriers, look at the Brenneke loads.

As for platforms/ammo to use these in:

Rifles: Quality AR-15's such as the Colt 6920, Noveske, LMT, etc. Stay away from 1/9" twist 223/5.56 if possible as 1/7" twist is better with heavier, more effective 75 and 77 gr. loads. The best loads in this caliber are the Hornady 5.56 and 223 77 Gr. TAP loads (the 5.56 load is higher pressure, higher velocity and gives you an edge at 300 meter) and the Black Hills Nosler 60 gr. You can also get barrier rounds from Federal and Hornady if you look around. These come in 55 and 62 grain loads. Either is fine. The new S&W M&P 15's are proving to be fine rifles, but they have only 1/9" twist. If you use them, you may want to stock up on the Blacks Hills 60 gr. Nosler load. The new Magpul Masada may prove excellent, but since Bushmaster will produce it, the jury is still out. Civilian versions of the FN-SCAR are supposed to be on the market later this year. Don't hold your breath.

6.8: There are MANY fine loads in this caliber. More info can be privided if you need it, but you can get good barrier loads here that are also good for hunting, self defence, etc. This is a fine caliber in AR-type rifles.

30 Carbine - This is a poor choice with FMJ, but loaded with Remington's 110gr. Soft Point it gives performance equal to or superior to even the best 223/5.56 loads out to 100 yards. In a self defence scenario, this is fine. It's great for small statured folks, is light in weight and with the extremely reliable 15 round magazines it doesn't have that "assualt rifle" look that scares the PHRASECENSOREDPOSTERSHOULDKNOWBETTER.. Fine rifles can be obtained from both Fulton Armory and MILTECH. Make sure you get one with the later edition sights, round bolt and 30 round mag release. Auto Ordnance is producing these rifles again, but the jury is still out as to the overall quality. By the way, the Remington 110 gr. SP load works better against auto bodies and windshields than almost any 5.56 load, while still offering acceptable penetration/expansion AFTER penetrating the intermediate material. Israeli reserve security forces still use the M1 Carbine for oviously good reasons.

308 - This will take any big game animal in the continental U.S. if you do your part. I wouldn't pick it as my first or even 5th choice for grizzlies, but if it's all I had, I can load it out to get the job done. The most effective ammunition for self defense is the 155 gr. Hornady A-Max match bullet loaded by both Hornady and Black Hills. Out to 600 yards it provides non-survivable wounds if placed in the chest cavity. The Accubond 165 and 180 grain loads are every accurate barrier penetrating loads that also work well on large game such as elk. Nosler Partitions in 150 grain are also excellent loads in 308 and on par with the 155 gr. A-Max. Rifles would include pre-ban H&K 91's, Springfield M1A's, and FN-FAL's. I prefer the latter and have significant trigger time on all three. DSArms makes fine FAL variants for nearly every need you can imagine. The various AR-platform 308's have all provem problematic to date. There may be better options on the horizon, but it's too soon to tell.

Handguns: 9mm: Glock 17 or 19, S&W M&P, H&K (various models), Springfield EMP, CZ or Armalite CZ-75 are all excellent handguns. (There are other fine 9mm pistols, but this list covers some pretty fine stuff.) Pick the one that fits your hand and you shoot the best. Everyone has a favorite. All those listed are excellent and will provide reliable service. Each has its faults and strengths, but they will all work well if you learn to shoot them and PRACTICE a LOT. GET PROPER TRAINING.

40 S&W - At this time I have to put the S&W M&P at the top of this list. Unlike the Glock 40's which have proven troublesome in this caliber, the M&P was designed at the outset for the 40 and has a longer frame life than the Glock by at least 10k rounds. They are also far more ergonomic to a majority of human hands. H&K also has some nice 40's, but the ergos on the Smith allow them to give less felt recoil than any other 40 on the market. This is subjective of course, but it is very noticeable in side-by-side comparisons. the interchangable back straps also allow them to fit nearly any hand.

45acp - Springfield, CZ, Nighthawk (ridiculously expensive) make fine 1911 pistols that usually work well from the box. 1911's tend to need tweaking. If going the custom route on 1911's (recommended), I'd start out with a Colt new model Series 70 (not the original Series 70) or Caspian frame/slide. Stay away from any 1911 with the Swarz safety, which means the S&W's and any Kimber with a "II" in it's name. For non-1911's, I'd look at H&K and the new S&W M&P. Glock 45's are problematic, irrespective of those on this forum who have one that's never given problems. They also don't fit too many hands well.

In pistol calibers, get the one you shoot the best. With modern JHP's, there's statistically little difference in effectiveness provided you use good quality JHP's. If you are forced to use FMJ, then go 45 since it is the only caliber listed that will create secondary missile wounds from crushed bones as it passes through. The 9 and 40 can't do this in FMJ. Almost anyone can shoot 9mm, and my 5'1" wife shoots 9mm and 45 acp very well, having competed some years back with a 45 auto.

Shotguns: This one is easy. Get a Remington 870 Police model. If you can afford it, get one customized by Hans Vang. Sorry, but the Mossberg and Benelli guns don't have a good track record when handled harshly (as in survial conditions). They may be fine on the range or in the hunting field, but when they start getting banged around and abused, the alloy receivers cause fits. As with all things, YMMV.

This only covers defensive firearms. If you want more traditonal hunting style rifles, etc., there are simply too many fine firearms out there to try to list.

For rimfire rifles, I like the Marlin Papoose for a take-down rifle or the out-of-print Marlin 39 TDS. The new Browning T-bolt is a nifty, lightweight bolt action 22 rifle that should be great for survival purposes, as would be nearly any Marlin, Savage, or CZ bolt gun, the Marlin semi-autos and the ubiquitous Ruger 10-22. I have an old Remington 541-T that shoots like a theodolite. It's my favorite squirrel rifle ever. Whatever you get, put good optics on it. They are at least as important as the rifle inself.

Others will have differing opinions, but if you stick with the firearms lised here, you won't be doing bad. There are many current firearms in vogue I did not mention due to problems being encountered in the field. The firearms I listed are those I know tend to work and work well. However, all of these are of course made by man and so you can get a lemon. And as with all things, YMMV.