This was tough as I've gone through too many, but I have to say, my old G35 was the most reliable car I ever owned, it handled nicely stock, and was fairly peppy (although very light on torque
)
My e46 M3 was a great car, although a bit slower than I expected with the SMG.
My 2000 S4 was probably the funnest modded car I had as it was very easy to modify.
My current 09 135 is a great car, but I hate the reliability factor of this car with a passion.