I think out if the box it's very hard to beet the sound of a csl at full chant, that induction noise is just pure savage.
Over on the ring last year a e92 gts and a gt3 rs 4.0 passed me by at well over 100mph and both sounded unreal.

If you start modifying exhausts I think the e60 with a race exhaust (akraprovic(spelling) eisenmann or tubi) would sound amazing, there's a YouTube vid somewhere of an m5 sounding like the old v10 f1 cars