The ranking system is a well kept secret for some reason, but diamond league is the top league. I'm nowhere near as good as the people who should be in diamond league, so I can only assume not many of the pros have been playing since patch 13 when diamond league was introduced. the current ranking system goes
Diamond
Platinum
Gold
Silver
Bronze
Each league has multiple divisions (groups of 100 players), and each division has ranks 1-100. People in Diamond league are exceptional players or rather they should be. You play a series of 5 placement matches to do determine your league, I won all 5 of my placements, but that was mainly because I played people who were terrible. Regardless of the reason, I've been jettisoned into diamond league where I am sure I will lose every match until I'm back down into platinum. Still makes for some cool bragging rights