In a semi-auto (or full auto) firearm they are the most critical single point of failure, outside ammunition. Of course most guns can fire without a mag or with a bad magazine, but that can make for a nightmare scenario if we are talking about your defensive or carry firearm.
For your fighting handgun or rifle you will want to make sure to pick reliable ammunition and to be sure to select a reliable magazine. Choosing a magazine should be simple, but modern marketing and pretty packaging might make it difficult for some to ignore the value priced stuff or confuse a sub-par magazine with a known name brand one.
I will attempt to point you in the right direction when it comes to magazines, but remember the most important rule: Magazines are a perishable, wear item. Do not become attached to them. Discard mags or place them on training only status when they start showing any signs of failure that cannot be corrected with replacement parts like spring and followers.
If you carry an Austrian wonder-nine or rely on one for home defense, stick with GLOCK magazines. This may change in the very near future, but any magazine other than from GLOCK for the GLOCK handgun should be relegated to range use only.
This one is easy, stick to Magpul. They are inexpensive and they work. You can also count on the ubiquitous USGI magazines as well as the USGI spec mags from D&H and Brownells. I would stay away from any other metal magazines, especially when you consider the fancy stainless steel magazines cost more than the PMAG or USGI mags. I can also recommend with no hesitation Lancer, TangoDown and Troy magazines, but these are usually more expensive than the Magpuls.
This list is short. Magpul. (If you have a platform that takes a different magazine pattern, you are probably wrong.)
The AK has so many variations and levels of quality and quality control, that it is a bit tricky to recommend a specific list of magazines for the platform in general. If you stick with surplus magazines such as Bakelites from Russia or steel mags from China, Yugoslavia or Hungary you will be good to go. I know these surplus magazines are not a plentiful as in the past, but they can be found for sale online. Be sure you are buying actual military surplus magazines and not inferior copies. Stay away from Polish polymer magazine and only buy Bulgarian magazines if they are “Circle 10”, and buy from a reputable vendor. Besides the Circle 10 mags there are newer polymer magazines from Tapco, US PALM and Magpul. While these US made magazines have not been around as long, they do enjoy a very solid reputation.
There are probably more magazines for the 1911 that work just beautifully, but with Chip McCormick and Bill Wilson making magazines I do not even bother to learn them.
When it comes to other handguns the easy choice is to stick with OEM factory magazines. I also can heartily recommend anything Mec-Gar produces. If you do your research a surprising number of OEM magazines come out of the Mec-Gar factory.
I can assure you, dear reader, that almost any attempt to purchase value-brand mags instead of the ones I have recommended will end in tears. Do not bet your life on a couple of dollars. Drop a line in the comments. We would love to hear from you.