Out of these three I've voted for Science Diet, because I'm afraid that Nutro doesn't offer a Hairball Control/Slim Diet and I am also not fond of IAMS...

But if your Kylie loves Royal Canine Slim Diet so much, why don't you stick with it!?
I've tried such a lot of different Hairball Control Diets on Nellie (because she has the hairball problem, too), but none of the diets really worked...so instead of changing her diet I would suggest to try an add some olive oil (or similar) to her food and give her malt paste...and of course a daily grooming session is helpful for the cat and fun for the purrson!