Mpeg 4 is an excellent format to use - it is an open standard, thus not controlled by Microsoft or Real and as such /should/ play on any media player that can decode mpeg4.
Windows Media version 9 and 10 actually use a distorted, fucked up form of mpeg4 that won't play on anything other than Windows Media Player.
You can wrap an mp4 file up into a Quicktime Movie wrapper so it has the extension .mov which wil always be associated with Quicktime player on both Mac and Windows. You need Quicktime Pro to do this, however.
I use Final Cut Pro and Compressor to make mpeg4 files then move them into Quicktime Pro to make them into .mov files, adding info and copyright info in the headers etc.
If you want to remain cross-platform but think mp4 is a bit too exotic, I would suggest Real format. As sucky as the company is, their Mac player does do what it says - play .rm files, which is more than I can say for the shitty, shitty port of Microsoft's player to the Mac which is very hit and miss.