"Best" place to buy ebooks?

I'd like to get information/opinions about the "best" place to buy ebooks.

Now that I have my first e-reader, I will be routinely adding new ebooks to my library. I had been using my tablet (Android with Moon+) as an e-reader, and sporadically getting ebooks from Gutenberg, Amazon, and various other sources. With the new e-reader (Note2), I anticipate a significant increase in ebook purchases. I currently use Calibre as my library manager, and routinely download any purchase and convert it to a standard epub format. This will continue, as I plan to add annotations to some/most files)

The genre's of interest cover:
- classics / "Great Books" / literature (e.g., Dickens, Austin, Lewis, etc.)
- Non-fiction (science)
- science fiction
- history
- biography
- etc. (it could be a very long list)

Does anyone have recommendations for the epub store that provides:
- high quality epubs
- verified translations or new releases of publications out of copyright?
- download capabilities
- format choice
- other factors that you have found to be significant?

I realize this is a fairly broad question; please don't hesitate to add clarifying factors.

Thanks in advance to your advice

Ed