WebScription.net. Useless if you don't like to read sci-fi, but if you do, they're great. DRM-free, choice of formats, $6 USD on average.
Smashwords I've used for buying indie works and backlisted works from published authors. Also DRM-free, choice of formats. Formatting can be a bit screwy, but not so bad as to affect the reading experience, and certainly not for all EPUBs either. Browsing is horrible, and Smashwords is best used if you already have a particular title in mind.
Borders AU has worked well for me, though I haven't used it recently and I don't know if being under new management has affected anything.
I haven't used Readings, but it's not worth considering, IMHO. The books you purchase from them are stored in the cloud and
read through a browser or a mobile/tablet app.