The later 2016 with AP 2.0 but MCU1 up until Jan 15th 2017 is correct... as they had free SC and connectivity... what makes for a unicorn is that the original owner purchased FSD. Now, they had to redeem the FSD, at which point Tesla would put in AP 3.0 and maybe MCU2. If still only MCU1, $1500 (or 1500€) is a small price to pay for that upgrade with such a unicorn for a Model S/X.
The thing is, the original owner MUST have redeemed the FSD. If they purchased it, but never redeemed it, policy is that Tesla will only redeem it for the original ownerc, so if they are selling and haven't had it redeemed/upgraded yet, make sure they do before the handover (before you purchase), because Tesla won't for you as the new owner.
As far as the best rarest most vauable combo, I would say a late 2016 Model S/X with free superchargig and AP3.0 FSD and MCU and connectivity is the best of all worlds and extemely rare, making for a newest tech car at a great price and free juice forever. Just not the fastest charging speeds anymore.