Louis- lou-ee or lou-is (I've known people with this name who pronounce it both ways)
Lewis- Lou-is
Luis- Lou-EES

I like both Louis and Lewis about equally. I'd use Lewis if you don't want any pronounciation issues, but I think people will get used to Louis however you want to pronounce it.

Luis is a tradtional spelling if you're Spanish or Hispanic. It only isn't traditional if you pronounce it like Lewis/Louis.