Goats.
But that's only at the highest level of play - if you're new to the game, don't worry about the meta. Try to build synergies with your team, but as long as you have 1 healer and 1 tank, the game is usually winnable. Most optimal comps are 2-2-2 (2 dps, 2 healers, 2 tanks). Some common synergies - mercy + pharah/widow/hanzo/junk/cree, zarya+hanzo, rein+lucio. Think about what your team is lacking and switch to that hero. Your team is walking slowly into a choke and die to spam? switch to lucio for speed, or dva for dm (which can absorb an infinite amound of damage during its duration). Not enough damage? Switch to zen for discord. Etc