AIFF or WAV. I'd use WAV.
Note that when you Export a track, any gap between the start of the recorded track and the zero seconds mark is discarded.
For example, if your recorded track is 10 seconds long, but it does not start until "time=5 seconds" and then continues to "time=15 seconds", when you export the track (using "Export Selection") the Exported file will be 10 seconds long (the 5 seconds of "nothing" is not exported).
The quick way round this is to add a little bit of silence (Generate menu > Silence) at the beginning of the track.
Silence is not "nothing" (it's samples at zero amplitude) so then the full 15 seconds (5 seconds of leading silence and 10 seconds of sound) will be Exported.