If you can afford it the Wersi OAA (Open Art Arranger) looks to be one of the easiest and most versatile, (This is by comparing the descriptions in the Owners manuals however, as I have not had the chance to try all the different manufactures style creation side by side) however some people may be put off as the manual runs to 60 pages. (Don’t be though, as the manual covers many other features besides style creation)
Also please remember that just like sounds, how the OS works is also personal preference. (Just because 1 person enthuses how good it is, others may have a different opinion)

Bill