I believe that AVI on PC and hi-res Quicktime on Mac are the most "lossless" formats.

MPEG is a higher compression format that stores full frames every so many and then changes to the frames, and I suspect that Real is the same way.

Not sure about Windows Media; I know it's a very high compression format at lower bitrates, but not sure about in higher resolution. And I'd stay away from SWF or FLV as a source media format.