the newest one. cause it has everything the old ones do and more. FL studio 5 is latest atm. 6 is coming out soon.

Yes get FLStudio 5 and then there is 3 different versions of it. A $99, $149 and i think a $299(these are the downloaded versions. There is boxed version but it costs more and you have to purchase the free update thing). Here is a comparison chart of what you get at their website FLStudio