Ball for QP
Really versatile hero in a competitive setting and when your team is trolling or not coordinating. Plus, you can always practice all the techs (double boops, 180° boops, wall jumps), efficient grapples, spawn boops, & rollouts.
Ana, Lucio, and Mcree/Ashe for the other roles. Lucio is also really fun to practice for the same reasons as ball. Lots of boops and efficient wall riding routes you can practice in QP to master and abuse in competitive.