What is the preferred ebook format?

Dear all,

I know the most ebooks out there are in pdf format, but as more and more ebooks are available in epub, which one is your preferable format? Just want to here your opinion.
Also, whenever I convert ebook to-and-from pdf & epub, formatting run crazy, anyone out there has good utility to do a better job? (I am using Calibre to do the conversion)

Trader T