The "best" real-time multiplayer platform for mobile games is code you write yourself in C++ to do exactly the things you need, on top of the native networking functions, and Apple's game kit, and the Android play services. This is when you define "best" as "uses the least resources, and integrates best with each mobile platform."

However, "best" often means something totally different, such as "makes it easy to ship a finished game with working multiplayer" or  "has built-in tools that are easy to use" or whatnot. You'll use an engine for that, and perhaps some add-on libraries, too. If you know how to develop in Unity and Gamesparks, then that is very likely the "best" choice for you using this second definition. Shipping working games is hard, no matter what platform you choose, and as long as you don't run into a fully unrecoverable "can't do that at all" problem, any platform is likely to make it easier than rolling your own.

Good luck with your game!