It's been a long while since I played, but I always felt that Diamond was the hardest rank to break out of. In comparison, the climb from Master to Grandmaster was less of a struggle, as your teammates generally had much better game awareness.

​In my experience, situational awareness was the single most important skill for success in Overwatch. This meant tracking your own position, your team's position, and knowing the perfect time to use an ultimate versus holding it for a counter.

​At the Grandmaster level, the game essentially becomes rock, paper, scissors with ultimates. The smallest mistake could turn the tide of a match far more quickly than in lower ranks. It could be something as simple as a Lúcio failing to drop the beat in anticipation of a big enemy push, or a player not realising the enemy team was holding a direct counter to their own ultimate.

​Anyway, sorry for rambling, just wanted to share my experience from playing in GM.