For me personally, there is one thing that defines a great IGL:
If you're spectating their team and they're down 9-3, at the back of your mind you still think "Winnable"
They are capable of keeping their team disciplined no matter the stakes, and the fact that they show an extreme affinity for bringing back rounds, an indicator of their ability to effectively counter-strat their opponents.
There's a reason everyone reading this is thinking of the same 5 people.
Also another reason why I think Steel is slept on as an IGL