If it's the same anti roll bars as the E46 uses then the stiffest one were found on the convertibles. There was four diameters on the front. Went thru this when I upgraded my Touring to sports suspension. Put a set of convertible ones on and it cornered something lovely.
Edit: Quick squizz at RealOEM and the non M3 E46 tops out at 24mm, while a 3.0 E85 is 25mm.