It depends on what I'm buying. For most of my makeup needs, Sephora is my go-to online shop; for skincare products, AbsoluteSkin is where I order; and for most of my yoga clothes like leggings and tops, The Upside is my online store of choice.
I don't really like buying clothes online in general because it's very unpredictable and it takes too much time.
I only shop online when I'm home in Australia, and on of the online shops I frequent is a brand named Cooper St. They have a really good collection of dresses and tops, and I personally like how feminine their designs are. The first thing I do though is check out what they have on sale, because I want to save as much as I can, and most of the stuff they have on sale are really good ones anyway. So, yeah, that's one of the online stores I like.