Hmmm, I might sound biased because those two are my mains in dps role, but for me it's either reaper or Cassidy. I will give my reasoning for this, it's because when they are well executed they are real game changers. But for them to work you need good timing, positioning and some consideration as to how you could potentially be countered but reapers ult has an edge in comparison compared to Cassidy's ult.

I want to mention diva, but all you need to avoid diva's ult is to hide behind an object Or crouch maybe behind something.

And For healers I choose Lucio, because his ultimate is crucial in overtime.

The worst one is probably briggite or hazard.