There's no single answer. It depends, in part, on the graphic itself, and
also on what you're planning to do with the document. In my experience, GIFs
are usually best. JPEGs are often (but not always) much smaller files, but
they won't stand much manipulation or re-sizing -- so the final result may
be lousy if the document is viewed on screen at a different size, but OK if
the document is intended only for printing.