By both volume and $$ amount spent, it goes:
- Fictionwise's DRM-free MultiFormat selections (surprising amount of good backlist sf/fantasy/mystery still popping up)
- Baen's DRM-free author reprint/other publisher bundles (not really a monthly Webscription buyer because I actually don't care for most of Baen's own currently published offerings)
- Kobo for DRM-ed "mainstream" books when they have the good discount coupons
- Direct from author/publisher when they're DRM-free and doing the special offers (this is probably actually tied with Kobo, since I haven't really gotten much from Kobo for quite some time while I've been buying more publisher-direct stuff)
- Occasionally Smashwords when there's a backlist title and the author is also offering the discount coupons
- Misc other DRM-ed stores with the special offers
I really only buy* e-books if they're exceptional in some way (exceptionally enjoyable writing, exceptionally low price w/exceptionally promising blurb, etc.).
Otherwise, there's the library, and I still spend most of my money on paper books (but for artbooks and foreign language stuff, and fiction comparatively rarely and usually only from my favourite authors/when the used bookshop or library sale is offering "new" stuff that I can try on the cheap).
* For the record, I've spent enough on e-books in this past year alone to re-purchase my e-reader at least twice over.