If I were buying today I'd be tempted with an M235i but wind back three years when I bought my z4 coupe then probably a 135i coupe, still think they look good with the right wheels.
It was early in 2003 when I purchased my first Z4 and at the time I was also looking at a Boxster so it very likely that had the Z4 not come along I would have purchased the Boxster. However I don't know if I would have liked the Boxster as much as I like the Z4 so I'm not sure what I would be driving today. I'm still very happy with my almost 10 year old ///M roadster and I have no plans to replace it anytime soon.