The easiest ebook format to strip DRM from is the Adobe DRM that various stores use. I only buy ebooks from the Google Play book store now, since DRM removal is so straightforward. (Same deDRM calibre plugin, but you just get a single encrypted ePub you can download, and adding it to calibre gives you a DRM-free ePub, which is a more industry-standard format. Converting to or from KFX or AZW3 can be error-prone.)
