The high-end sites are the best; NAP, Matches, My-Wardrobe (although Matches always says everything is true to size, which I know from ordering certain brands is not true)but I guess they invest a lot as some of their clientele spend an absolute fortune and not just at the end of the sale with a discount code, ahem.
I like any site with that publishes reviews, even if you have to read some with an open mind, and ones where you can scroll over the image and get a good close up or zoom in of the fabric. J Crew is a good example of both those.
I hate the way Baukjen displays stuff now, e.g. click tops and its a messed up mix of styles and colours, rather than one top after another. I quite like that Boden lets you choose to see all colours, or just one, for this reason.
My award for worst filter ever is Russell and Bromley. Who on earth want to filter by heel heights 0.5cm increments??? Thats the worst one, but all their search and filter options are rubbish and it doesn't remember what you do if you press back for whatever reason so you have to start again.