Mytheresa is becoming my favourite online retailer, they have the best of everything, far far better selection than any retailer,
Luisa via Roma never disappoints , excellent customer service, and they send out amazing codes, they recently added kids wear which is a huge life saver for me as I buy a big chunk of my kids clothes online, very easy returns,
matchesfashion.com -- great customer service, decent selection (could be better), good user interface, speedy international delivery, and those awesome boxes! .
Matches used to be my go to boutique, it's very close to where I live in London, very good service and decent selection .up to a year ago, they were on the top, but something must have changed in management as its nothing like it used to be, the selection is very poor lately,
Net-a-porter is good for emergency deliveries specially if you live in London, a few hours and it's delivered to your home, but their selection of late is completely off putting,