Forewarning, I have the S6, not S7. It's my DD (~25k miles/yr) and hasn't been to the track. I think a lot depends on personal preference & intended use, but here's my $0.02:
I have ACC and I wouldn't want another car without it. I use it almost all the time, both for the convenience and safety. Every time that some bonehead does something that seems like they are trying to kill me (sudden lane change, run red light, pull out without looking, panic stop because there is a deer on the side of the road, etc), the ACC beats me to the brakes every time. I believe it was part of the Driver Assistance package & Innovation package, depending on year.
I like the comfort seats. I sweat a lot & run the ventilated seats almost year round. Audi's not as good as some others, but still I wouldn't want a car without it. When I got it, I thought the massage functions were a gimmick, but turns out I use them daily - reduces stiffness especially on longer drives.
I briefly drove a BMW with HUD and do miss it a little. I believe depending on year, was part of the Innovation package as was the night vision.
I think the LED headlights are worth searching for.
I have the Bose, which isn't great, but frankly if I'm looking for great audio, I don't go to a car. B&O in cars I test drove was definitely better. I'm considering doing the subwoofer mod others have mentioned.
Google for "Audi order guide" for the model year was a big help to me in comparing what package/option I needed to get what I was looking for back when I was shopping. Of course, there were fewer C7 model years available back then ;)
Good luck!
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