iMovie will import anything that Quicktime understands, as far as I know. The other day, for example, I was trying to import a MPEG-1 encoded video.
The encoding speed is an issue, though. I gave up once I lost interest in what I was going to do with it since it was taking a _long_ time.
I would assume that DV would work best since that is the iMovie native data format for clips. I am not sure if it would just copy them or actually force a re-encoding into a format it wanted, though.