I have stayed in South Korea on and off for over 16 months and I have tried a majority of Korean beauty brands. There are far too many brands here but the packaging, product concept is similar in many brands whether it is face masks, hand creams, lip crayons, face washes. Many brands have the same amount of product -> 300 ml face washes in Etude House, the Face Shop, Nature Republic etc. and in a similar price range. So let me share some knowledge to help you decide how to choose products from Korean beauty brands.

I have tried drugstore brands which are in the low to medium range category. The brands that you can find in Korea are so many that this post would be thousand pages long if I write them all but I will write about the ones that you can easily find in Korea and are easy on the pocket too!
So here is a list of the easily available, affordable (low to medium price range), good-quality Korean beauty brands:
The Face Shop,
Etude House,
Nature Republic,
Missha,
Holika Holika,
It’s Skin,
Innisfree,
Skinfood,
Aritaum,
Mamonde,
LG: On the Body
Beyond,
VDL,
Tony Moly,
Too Cool for School,
Shara Shara
These are the brands from which I have tried something or the other over the course of one and a half year. The most easily available products are from The Face Shop, Etude house and Nature Republic. And these three are usually located very close to each other in every shopping area (even smaller ones) all over Korea! These three brands also have regular Sales (monthly!) and always come out with new products all the time. The packaging and quality is at par with other expensive brands.
Most of my makeup and skincare collection is from these three brands and I have my favorites from each of these.
I love hand creams and lip balms from Etude house, sheet masks and nail paints from The Face Shop, BB creams and face massage creams from Nature Republic.
Missha products are a little more expensive than the trio I mentioned above but they have the cutest eyeshadow duos, trios and quads; they are small and compact and have very nice color combinations. I also like Mishha Face primer, mineral powder foundation and BB cream!
Since most people want to try our BB creams and sheet makss from Korean brands, I have a couple of suggestions:
1. For trying out sheet masks from korean brands, look out deals like 10+10 or 5+5 and in most of these cases, you can select a different flavour (?) for the free gift. I did it with my The Face Shop 10+10 offer. I picked 10 different sheet masks and got 5 Blueberry + 5 Rie face masks. For Holika Holika sheet mask 10+10 (pack) offer, I picked 10 of Aloe and got 10 Honey face masks (free). Of course you have to ask the sales executive first!
2. For BB creams, do not try them on your hands or pick out shades that you think might match your skintones. Most BB creams in Korea do not come in many shades but the lightest one which would look like made for a very fair skintone might blend well in a wheatish skin color. I and my close friend always shop together – we are different in skintones by at least 3 shades and yet we wear the same shade of Nature Republic BB cream! So always try out BB cream on your face and wait for at least 5 – 10 minutes before buying, they look better after settling in for a few minutes!
3. As per my experience, It’s Skin has some of the best skin serums and sunscreens!
Korean beauty products have been a hit and miss for me in many cases just like any other brand from any part of the world so do not buy something just because it is from Korean Beauty Brand. I do think some products are revolutionary (like Snail skin cream or Placenta face mask even though these are not my cup of tea!) I definitely think that other  things from Korean brands can be found all over the world from non-Korean brands just as well. Again this is my opinion, to each his own. My only advice would be – do your research especially if you are going to be ordering online!