I recently used @Pubnub to implement a realtime leaderboard into my @Unity game. It was easy and straight forward to get up and running. I choose Pubnub because they had a @Unity SDK and an example project which showed me how to use it. I downloaded the example project and tweaked it to fit my game.

In their documentation I found good information about what I needed to know. Also, their support is very helpful when you have questions.