Generally, DV AVI is a good format to retain high quality. Also, AVI with certain (e.g. lossless) video codecs. However, the latter is far less compatible and useful than DV AVI. However, since you're asking the question specifically pertaining to the upload of video material as streaming video to the Magix online services, you would have to limit yourself to the formats supported by the encoder on the server. You should definitely be aware of the fact that quality losses cannot be avoided here, as you will be working with highly compressed video codecs with relatively low bit rates. I can't really say that I have ever done something like a comparison test as to what upload format helps to retain the best quality after it is converted to Flash video on the server, but I suggest you compare between the following formats:
- AVI with Motion JPEG codec
- MP4 with H.264 codec
Especially the last one might be worth a shot, as I assume you haven't tried it before. Make sure to set your Video Deluxe MPEG-4 encoder to H.264 codec when exporting the MP4 file. The resulting file might not require any re-encoding on the server since FLV also uses H.264.