Best M3:
e30 - to me, there is something about those boxier, sqared-off edges that makes the car cool.
Best M5:
e39 - sure, the e60 has more power, but the e39 is much more of a sleeper, plus I jsut think it looks more distinctive, in a way,
Best Series: 3
3 series allows the most people to enjoy the essence of BMW. that's something great.