What wheel size is best?

[c.anders]

I've decided what wheels that i ideally want to buy (kansei knps) but i need to know what size they should be. For now I've decided on 17x9 because thats what i think looks best on a couple cars i saw on the fitness industries gallery but i want to know what works best for others.

 

[bmart]

For what car? Any mods done? For what use? Define "best." In general, OEM sizes work best across varying conditions.