I'd say #6 is a favourite of mine. It has the recipes for making bath products and lots of mixes. It was also the first book I bought, which is what got me hooked! I'd have to look through the others.
You wouldn't believe how hard it was for me to find #1 when I was collecting them. Even Gooseberry Patch couldn't tell me where to find a copy. I think its been reprinted since then.