I like mytheresa.com for their selection and availability. They carry a wide range of styles and their size range, specially for shoes, is better than Net-A-Porter. Their customer service is excellent also. I get responses and returns are processed quickly. I also receive my orders quickly (w/in 3 days of ordering). I am in the US, by the way, which is relevant for my next point.
My only issue is their shipping cost (45 Euros). It's quite expensive: probably because of the really nice packaging. (I wish they would give customers the option as it currently doesn't seem ecofriendly.) And depending on the price of your item, DHL may charge you for customs fees as mytheresa doesn't ship duty paid. These shipping companies charge for customs and their services, so you generally get overcharged. For this reason, if I see the same item at NAP UK for a similar price, I would rather get it from them. That being said, mytheresa.com usually has better prices than NAP.