In heists, every player, at the end of every round, is given a rank: Platinum, Gold, Silver, or Bronze.
What determines what rank you get?
As far as I know, the exact formula is well hidden inside the game code, but generally, I'd say the following activities definitely contribute positively towards your ranking:
Killing enemies.
Causing damage (for instance, damaging Lester's car in the first heist mission is known to give people platinum).
Completing mission objectives.