For me personally, it has to be Omen's ult since you can literally teleport anywhere on the map and change how the game is going. You can scout a site that your team wants to push and see if the opposing team is guarding that site or not. Y...

To the best of my knowledge Jett is the only agent that can use her ultimate, die, get revived and still have her ultimate. Also, her knives should have a timer feature like Reyna's ultimate. Replenishing after each kill she gets.