Here are my faves:
-The Face Shop West Covina, Monterey Park, and/or Arcadia. All three locations feature great sales, great staff, and always give amazing samples. I avoid the face shops in KTown, thanks to the horrid service I've always received.
-Maneki Neko Arcadia and Little Tokyo. I prefer the Arcadia store a little more, just due to the fact that LT can be super crowded and tends to sell less cosmetics and more touristy items.
-Image Beauty Monterey Park. This is a great store with an awesome mix of Korea, Taiwanese, and Japanese products. They have a huge selection of 3CE, Etude House, It's Skin, etc. The staff is super helpful and friendly and they are very generous with the samples
-Aritaum San Gabriel (In San Gabriel Square shopping center). They carry Aritaum, Laneige, IOPE, and Mamonde cosmetics, as well as their skincare lines and they are very friendly and are also very generous with GWP and samples.
-Beauty Concerto Koreatown Plaza. Great selection of lower and higher end Korean brands like Tony Moly, IPKN, Sulwhasoo, etc. They usually always have a sale in store and deals on sheet masks, etc. You usually have to ask for samples at check out, but they are very nice about it.
-Palace Beauty Galleria. This place is great for Japanese and Korean makeup and skincare, as well as more 'salon' based products. The owner is wonderful and super friendly and helpful. Service when the owner isn't there can be hit or miss, but as for product selection, they can't be beat.
-Amore/Aritaum Koreatown Plaza. Another amazing Amore/Aritaum store. The sales women are so incredibly friendly and are willing to help you find products without pushing products on you. They always have great sales and give the most samples of any of the above stores I've listed.
-Beauty Tips Monterey Park. A great spot for Taiwanese brands like Solone, Naruko, Bebe Poshe, etc. They also sell some Korean and Japanese brands, but their real focus is Taiwanese brands.
Once I think of more shops, I'll add to this list. I hope this helped!!