Overall Aruba is the windiest (but the high rise hotels are blocking more and more of the wind). Then Curacao has a bit less wind, but very nice slalom conditions and less tourism, more culture. Bonaire is the least windiest of the three islands (2 knots in general compared to Curacao). But is has Lac Bay where Freestyle conditions are perfect.
May is overall the most chance of wind. From august the wind dies until december.