A client wants me to provide quicktime files so they can edit in iMovie 6.0.3. Does it need to be a certain conversion of quicktime? What is the easiest to deal with?

That's an old version of iMovie, goes back to '06. The potential issue here is that if they are operating on an old system that has not had it's QT components updated the files you give them might not work properly.

Assuming you and the client are working with the at least QT 7 and you're giving them a format that matches what they are working with (DV or HDV) then you should be fine.

Unless they're working with an older Mac that isn't compatible with the Intel-required software I'd suggest to your client they upgrade to iMovie '09; far more options and formats and the editing capabilities went through the roof compared to '06.