For a detailed comparison see
eReader for the Hebrew reader. The list of Arabic compatible devices isn't necessarily exactly the same as for Hebrew, but it should be close. What I get from this thread is: a) Linux-based readers that include FBReader support RTL languages and this includes PocketBook and probably Kindles with
FBreader & fbKindle, b) Android-based readers with Aldiko or Moon+ (but not FBReaderJ) support RTL languages, and c) Apple iThing. I have not seen anything one way or the other about the latest version of Aldiko (version 2.0), which includes Adobe Digital Editions and so might have dropped RTL support. If Aldiko 2.0 does work, it will be the first app for DRMed Adobe ePubs that supports RTL languages.
Finally, PDFs with embedded fonts that are formatted for a 6" screen (say) are the most widely applicable way to read RTL on ebook readers. Calibre can convert from MOBI or ePub to PDF, see
Hebrew pdf output for kindle - newbie.