Some of this is a repeat from earlier posts, but here goes with my experience:
My is S8 currently shod with a mix of 2 Michelin Supersports (front) and 2 Michelin PS2s (rear). Supersports are definitely grippier in the dry, and I can't say I've found them any worse in the wet. My "spare" 20 inch wheels have Dunlop SP Sport Maxxs all round ( I think...), but as I've not had these fitted on the car yet, I can't comment - though i would, of course, be interested in the experience of others of Dunlops
.
The only other tyres I've used which have been mentioned here by Zip/Neil are Toyo Proxes and Pirelli P Zeros, on my Fiat Coupe. I agree with the comment about the P Zeros being a good all rounder, but I've found the Proxes grippier, though they tend not to go out in the wet very often these days..
, and I'm not sure if the experience with a different car/tyre size is all that relevant.