I don't live in Bangkok, so can't really compare them to other health food stores in the area, but Radiance does have a pretty good selection, especially considering how tiny their store is. Loads of varieties of dried beans and lentils, nuts, nut butters, seeds of all types, nutritional yeast, supplements, teas, brown and mixed-grain rice, toiletries and hygiene products, and a small selection of fresh, local, organic veggies.
The prices are about as expensive as you would expect for imported and organic things. The peanut butter is a great dealt though!
Oh, and the owner speaks good English and can probably help you find anything else you need.
Note on the location: Radiance is located directly on Soi 22, just about one kilometer south of Sukhumvit. You do NOT
need to go down the Nam Thip Alley 2. It'll be on the right side of Soi 22 if you're headed southwards, just after a couple of schools.
Cons: Not much fresh stuff