E39.
Reason #1 I still love the styling, especially the post-'01 facelift with the angel eyes and clear corners. It's what a proper BMW looks like.
Reason #2 It was made into a police car when the Need for Speed series first got into the "pursuit" mentality. V8 power, heavy (great for road blocks and pit maneuvres), handles well, 4-doors and room in the back for hauling 3 drunk drivers to lock-up. Perfect.
Reason #3 That Guy Ritchie BMW Film posted above.
I liked looking at it so much that when the lease ran out on the '04 E46 330i ZHP, I was seriously trying to find either an E39 540i M Sport 6MT (which proved elusive) for $36k CDN, or I found an '01 E39 M5 which had done 50k km for $50k CDN, black on black with wood trim.
Filing through the service record was a bit scary, the previous owner spent $6k on it over the course of 1 year after the warranty ran out, having a car that would have been 10 years old when I finished the payment was disheartening, the cup-holder didn't retract properly and the pre-owned financing rate was rubbish. So, for the exact same money, I went for the brand-spankin'-new E90 335i instead.
Doesn't stop me from staring at it every time I see one, though.... If that had been an '03, so covered by the Certified Pre-owned Warranty, well, I suppose I'd be driving it still.