It all comes down to tastes really...but in my opinion
16" might offer a great riding comfort and no tramlining but most of them really sink into the body of the Z3 and doesn't enhance those curvy flares, especially for 2.8 pre and restyling. They look instead well balanced on a 4 pots pre.
17" look still OEM, the staggered ones like 18,32 and 42 are IMO the best, but the comfort deteriorates quickly and tramlining is noticeable
18" might look the best: I say might because if you go for 18" you must chose a wheel design fairly "full" as I believe those thin spokes that leave a lot of gap aren't really flattering towards the small braking system, plus some of them really makes the little Z3 look like a bandwagon...those of Antm72 or the classic BBS LM look good without the "bandwagon" effect.
19" are way too big for the car in my opinion...leave them to Xibit and Pimp my ride.