Baen, Kobo, Amazon, Gutenberg (in various countries), libraries, Feedbooks, Google. It all depends on where I can get the better deal or in the case of Gutenberg, which locale has the better version since I've found some odd formatting differences at times.
Baen makes the list since I often buy the monthly bundle which has 6-7 books of which 4 are new releases and you get the 1/2 book and 3/4 book releases of the new books prior to the release date. There is also the Grantville Gazette where I pick up the 6 book bundles so a year at a time (What can I say? I'm addicted to the ebook version of a soap opera
).
The public library is often the best choice for quality and price and would be much higher on my list but do I want to be #23 on the wait list for popular books?