I am not quite sure on a more preferred favorable format, other than MOV, AVI which works great for me.
They're some Advanced users on this forum, that might chime in, who can answer in detail on that.
Here are a couple links that will explain more on importing video that i hope can help.
iMovie '09, '11 supports most commonly-used video codecs, including DV, MPEG-2, MPEG-4, H.264, and AIC.
At the top of this forum, there are also helpful links under, Helpful Apple Support Resources, that might help you with more of your imovie project. I hope it all works out for you, and that this helps. Sidekick
Do you (or anyone else) know if iMovie can prompt you to delete the video files off of the phone (as iPhoto does) before dismounting the device? Otherwise, it seems that you are then forced to manually do so by hand in the cameral roll on the device.
Additionally, I learned from this video: http://www.youtube.com/watch?v=5UyHksf02bs (about 1/2 way in)...
...that copying video events within iMovie (from drive to drive) can be done by clicking and dragging the event, but by holding down the command button while doing so truly moves the project without leaving the original copy in place. This is what I would really like to do in iPhoto. "Move" the videos from iPhoto to iMovie so I don't have to keep track of each copy session and then go back and delete.
I've tried holding command with no luck. Also tried cut and paste, but it won't paste into iMovie. You can certainly "move to trash", but it sure would be nice if it had a "move to iMovie" option.
I think I found one way to do this, but since I have over 1,500 movie clips in iPhoto I may hold off until I get some secondary opinions and hopefully alternate options...
I created an album in iPhoto called "Move to iMovie". Then, I have a smart album in iPhoto with criteria to capture all MOV and MP4 files. This gathers all movies together.
Now, I'm able to copy the movies from the smart album over to the standard album. From the standard album, I could copy all of the videos (or in sections) over to iMovie by dragging from app to app. Then, I could delete all (or in sections) of the movies files from within the album in iPhoto to the trash...realizing one cannot delete files from a smart album.
I think this process would work, but I'm hesitant to try and move too many movie files at once, so I think I'll plan to do this in batches. That is unless someone here has any other ideas?
OK, so here's an update.
I found the only way that i could get it to work was to quit iMovie, then reopen it. On opening, it will ask if you want to import iPhoto movie thumbnails. Say yes, and after a few minutes the new iPhoto movies will be in the events pane of iMovie. Seems like a crazy inefficient system, but the drag and drop doesn't seem to work in v 10.6.8 with iMovie 09.