Two characters with the most synergy [closed]

I have this friend who I like to play Overwatch with.

Normally we just pick random heroes that our team may need or heroes we like playing. We are starting to play a bit more and getting better so we are starting to work more on teamwork, and we are starting to think more critically about our character selection.

Because there could be countless strategies between the different game types, maps, team composition, etc, I would like to focus on the general goal of killing lots of enemies. What pairs of heroes combine well to successfully take out multiple enemies (either all at the same time or survive long enough to take them out one at a time)?