SCART is a multi-purpose connection system and, depending on the unit to which it is connected, can transfer video signals (and stereo audio) to any of the following standards:
'Composite' - Video (only, not audio as well) all down one single line so generally accepted to be limited quality. Sometimes called 'PAL' or just 'Video'.
'S-video' - The high-definition monochrome signal and lesser-definition colour signals are carried on separate lines, with their own earths. Can also be transferred by 4-pin mini-DIN plugs/sockets. Generally accepted to be much surperior to composite video.
'RGB' - the three primary colours are passed along 3 separate lines. Generally accepted to be superior to S-video but not by a huge amount.
In the SCART system, composite video is always available, in both directions.
If enabled within the unit(s), and configured typically via the Set-up menus, you may also have /either/ S-video /or/ RGB - but not both - and they will be either IN or OUT, but not both.
So to answer your question: the best connection for viewing Sky is RGB; connect via SCART. But you must configure the box to output RGB, otherwise you'll get only Composite video.
Other units such as a DVD player may allow you to output (e.g.) RGB via the SCART plug (needs to be configured) /and/ S-video (to another unit) from the additional S-video socket. HTH.