What are your top 2 or 3 favorite sources for purchased books in epub format? For each, I'm interested to know the main pros and cons as you see it.

Mine:

Source: Google Play Books
Pros: Decent Epub 3 support and book formatting quality; good selection; Android/iOS/browser apps; easy download/import-to-Calibre via ADE; uploaded books get full support for sync of positions/notes/highlights; notes saved to Google docs and easy to export
Cons: Selection and pricing often inferior to Amazon; Android app somewhat less polished than Amazon's; no support for dedicated simple e-ink devices such as Kindle or Kobo

Source: Kobo/Walmart Books
Pros: Epub 3 support; good selection; Android/iOS/PC/MacOS apps; more competitive pricing
Cons: Selection inferior to Amazon; no browser app; Android and PC apps less polished than Kindle; proprietary .kepub file format requires extra conversion steps; no support for uploading your own books or synching reading data among them

In the past I would've included Barnes and Noble due to good selection, a good set of free apps, and sometimes competitive pricing. But their policies lately seem like they've hardened into trying to force you to use their devices and to keep the books you buy completely within their apps and ecosystem. Also, I'm interested in ebooks.com but haven't used them yet, like to hear if anyone has used them much.

Hoping that with the new Walmart/Kobo partnership, Walmart will invest and build up Kobo's store and device strategy. For instance, increase their selection, widen out their cloud service features for reading, and do more user-friendly things like allow users to upload their own books into the Kobo cloud and sync reading data. It seems like this partnership would have the potential to really offer a viable alternative to Amazon's monopoly and closed practices.