I just voted 3-5 immediately (BooksOnBoard, Fictionwise, Nook, Amazon, Kobo).
Looking at the posts though, that reminded me that I've bought the odd book or two from a couple different places:
- Harlequin (I think I had a monthly sub at one point)
- Humble Bundle
- Google Play
- Smashwords
- AllRomance
- Diesel
- several direct from publishe (mostly comics and manga)
I also found a PDB ebook that I didn't purchase from Fictionwise (my Fictionwise ebooks seemed to have all migrated to B&N). Not sure where I got that from. I think it was from eReader but not really sure.