M3 - E30
The point of the M badge is to denote dedicated attention to performance. The E30 M3 is the best example because the chassis is already pretty stripped-down, and you don't get wood grain or an automatic until the E36. It's a go-kart with a roof, it's stayed truer to its roots than the newer models.
M5 - E34
Apart from being the best-looking 5-series ever made, the E34 isn't as flabby as the E39 or that other chassis I forbid my hands to type. I know the newer cars produce way more power, but they're using gigantic motors (by BMW standards anyway) and have become too cushy and round. The E34 is a sports car where the others are luxury sedans.
Series - 7
They've yet to make a 7 I wouldn't love to spend a year's salary on. The 7-series has always been a sexiful combination of elitist Jag-ish luxury and incredible earth-shaking power. They've always stood right on the line between a romantic dinner and sex; spacious and accomodating, with the heart of a musclebound champion. They even made them with turbochargers and manuals in Europe!
CSL - Huh?
Did they actually make a lightweight E30 M3? I thought it was just the E36 and E46 for the 3-series..in which case I'd vote for the E46, because as much disdain as I harbor for their size, and even though it weighs three thousand pounds with a cardboard trunk floor, it's just so fast.