The free and open e-book standard EPUB is my preferred format.
I'll accept .mobi or .azw if an EPUB version is unavailable. But then I'll quickly convert them to EPUB so I can read them in iBooks.
PDF can be the only format available for books that contain illustrations and special layouts.
Personally i prefer Epub or Mobi formats. I used to only read mobi for my kindle however i now prefer Epub on the Ibooks app. As i used the kindle for most of my reading i had a few bad ecperiences with PDF formats that have made me swear off PDFs 
►SUPPORT AUTHORS. IF YOU LIKE IT, BUY IT.
►If you like a post, please use the
button!
►If you like a post, please use the
button! @arkonen: I, too, have tended to stay away from PDFs. However, I've noticed recently that texts and other books that were designed particularly for PDF are managed well in my iPad2. Illustrations and figures show as they should, the text is clear and clean, etc.
arkonen wrote:Personally i prefer Epub or Mobi formats. I used to only read mobi for my kindle however i now prefer Epub on the Ibooks app. As i used the kindle for most of my reading i had a few bad ecperiences with PDF formats that have made me swear off PDFs
Same here actually. MOBI for Kindle, ePUB for iPad. Will also occasionally read a PDF on the iPad too.
Wsb wrote:An issue I have faced here is that when I try to upload a original epub, moderators wont allow it if the forum has a PDF copy, which I think it really unfair by the users.
I think it's more a case of there is no duplication of releases allowed. If you post a book in ePUB when there is already a MOBI (or PDF....) release on the boards that means we have two threads for the same book.