I would normally say 5 series, but Bangle ruined the current one. The E39 is my ideal of what a perfect sedan is though. Screw anything Mercedes, Maserati, Aston Martin, etc puts out. The e39 5 series is perfection.
Now, there's no great one so I go with the 3 series cuz it's the least weird looking and has a wide variety of nice models (sedans, coupes, and converts).