24-bit PCM (.aiff, .wav, etc.) files at the project's native sample rate without excessive ('for volume') buss compression or any master buss limiting. A couple dB of headroom is always nice...
But finish the mixes -- If something needs compression or EQ, the compress or EQ it. If something needs reverb, put some on it (of course, try not to use too much, but that's a given). If there's something making noise on a track while it's not in use, edit it out or gate it.
You should be happy with the mixes "as is" before they go to the mastering guy (or gal).