M3, E30 Just for looks and balance.
M5, E60 gets my vote for being an all round performance car with 4 seats.
5 Series is the best as you will struggle to get a better car bang for your buck.
850CSi, you obviously haven't had one. They look nice, but they are a money pit when they break down. There is virtually no common part with any other BMW. Any 8 series in fact.
My favourite one is the one I'm driving on the day.