mpeg streamclip, how to choose quality???

The only option to preserve video and audio quality is DV Stream export from MPEG Streamclip.
Use that new file in iMovie, iDVD or QuickTime Pro to export to your intended playback format.

The "native" format of iMovie is DV (.dv) but iMovie 6 also allows a few new Project formats such as MPEG-4.
Since your file didn't "start" as MPEG-4 a DV Stream export from Streamclip would be a better choice to preserve quality and prevent iMovie from converting during import.

I would like to know for getting the best quality of the clip ( and yes I know for better quality I should probably get another camera..) which option to use of export file to quicktime, DV, AVI,...bla bla bla... convert or demuxe.

As Kirk indicated, you really have no choice in compression format if you plan to import the material into iMovie. If you put it in any other format like AVI, then iMovie will again convert it to DV from whatever format you use. Each conversion tends to lose a bit more of the original data. DV is a relatively low compression format (large files). You can use any of the StreamClip "Export DV" options (DV(DV25), DVCPRO25, or DVCPRO50) depending on how large you want the files to end up. You cannot "improve" quality per sé, but you can keep the loss to minimum. You can also adjust brightness, contrast, saturation, and/or volume during the conversion process. If the files are to be play on the computer, you may also want to deinterlace and/or crop your display area to get rid of the "overscan" that would not be seen on a TV but is visible on the monitor. Also, if you are using an old version of iMovie, be sure to set Streamclip to segmentize the files.


EDITED: Looks like you guys were talking as I was still writing.

Hmmmm...
Never tried it.
QuickTime 7 now allows multiple exports (even from the same file). I don't know about Streamclip.