Sony -- my first ebook reader was a Sony so most of my library is already collected there.
I like Smashwords for easily getting books onto my phone (Aldiko) and iPod Touch (stanza).
I was using Kobo for awhile, but with agency pricing, it isn't any cheaper than Sony anymore so I've gone back to Sony. I also have an easier time downloading Sony purchases; it seems the Sony software likes me better than ADE, which only works sometimes.
I don't buy from B&N at all and I only get freebies from Amazon. I'm passively boycotting ADE.
