DRM-free is best if you can.
Kindle is
very tricky to remove DRM from. Barnes & Noble requires either a physical device or the no-longer-available app.
Google Play has had a
lot of
issues so you may also wish to avoid that.
eBooks.com and Kobo are reliable. Either you'll get a DRM-free book, or Adobe ACSM which is one of the easiest to remove with
NoDRM's fork of DeDRM.