I've been to busy with other things in game to really compare all the BMWs. However I know for just being a shear blast to drive my vote goes to the M5 '05 with a fully modified engine and Drivetrain (no LSD) on race softs with
NO other modifications. The stock suspension is surprisingly good, possibly the best in the game, and even with the race softs you have to have good throttle control or its like driving a superheavyweight yellowbird. Watch your throttle control and that big barge just floats around the track and while I've never tested its top speed 175 mph without a draft on the Supercar Eifel straight is nothing to complain about.