Lucio for sure. Up until a couple months ago, he was the only character in pro play to have a 100% pick rate amongst teams.
Pro play doesn't obviously apply here, but it doesn't change the fact Lucio is literally irreplaceable because of his speed boost and is always a good pick no matter the comp. He can work with dive, more defensivecentric teams and more...unorganized comps.
Other characters i'd recommend:
D.Va - another great swiss army knife pick. She gels with pretty much any comp and can help support more barrier centric teams with her defense matrix
Rein - Another totally viable pick in comp because he does what few can do with his kit.
Soldier 76: Nerfed a tad recently but is still probably one of the better DPS in the game. Great mid range pick, solid ult and great mobility
Zenyatta: A little bit tougher to play due to his squishy health, lack of total self heal (outside of his shields regenerating) and lesser healing output, but he is an absolute monster if played well. Discord orbs help shred everything and his harmony orbs let him heal without putting himself right in the crossfire. Probably also has the best defensive ult in the game