PDFs for books are nice. I prefer AVI to MKV movie files even though MKV has more features, just because VLC doesn't work so great with MKV. For audio it's definitely V0 MP3 or FLAC; usually MP3 just to save space. For documents, RTF is far superior to anything else I've seen. DOC & DOCX files have so many compatibility issues even between different editions of Word that I've long since stopped bothering to ever use them.