Depends what you mean by best, I would imagine sound quality is similar between all head units, however obviously there’s various options regarding CD or cassette players, double or single DIN units and soft or hard touch buttons.
Not sure about how cross compatible they are, I would assume it’s just plug and play (However I have heard that players from CAN bus cars possibly might need an adapter to be used in non CAN bus cars and vice versa)
My favourite is probably the Chorus/ Concert mk1 (cassette and single central dial) as they are hard touch and I like the aesthetics.
I believe all are compatible with Bluetooth adapters, that work by mimicking the CD changer, but if the car didn’t have a changer from factory then this will need to be coded in via VCDS
(If you want a ‘factory’ bluetooth option then I believe you have to have an RNSE head unit and use a Bluetooth module from an A3)
In terms of BOSE I haven’t done it myself but I think you need the Bose amp, subwoofers, and appropriate wiring loom. And again similar to installing Bluetooth or CD changer, the car will need coding via VCDS