A tough question to answer. Everything you listed is important. But if I had to pick one thing upon which my enjoyment of an RPG hinges, it would be the action-resolution system, including combat. The rules which allow PCs to do stuff.
Even the Storyteller system, an ostensibly RP-intensive game, had a good action-resolution system (even if it did involve rolling giant masses of dice).
And why am I reminded of the infamous question, "What is best in life?"