Most definitely ZARA. They have such unique and affordable pieces. When they have sales you get such good prices and they have minimalist and trendy things so you get the best of both worlds!
J.Crew Factory without a doubt!! You get the same quality as you do at J.Crew for half the price! Usually you can find all your favorite pieces you've been longing for on sale here. It is my happy place
ebay is def #1, then amazon, poshmark, therealreal, lollipuff, etc + I have an extensive list of online stores where I can find Loubs and other designer stuff. Usually, if I am looking for a specific pair (it's usually shoes I am looking for, anyway), I will go through the whole list and check every website (like shopharper, ekseption, etc) - sometimes it is cheaper to purchase from abroad than locally (state taxes and vat)
I prefer to buy clothes the old fashioned way as I an be sure it suits/fits me before I buy! But Space NK I love for their choice and lots of my staples come from them!
Amazon for me. I only had Amazon gift cards on my wedding registry! I didn't have time to pick and choose what I needed or wanted anyway, cash is a lot easier for me to handle.
MrPorter and Net-a-Porter for luxury purchases, received good customer service so far. For apparel I like Weekday, COS and ASOS most, just because they offer fashion forward apparel for good prices.