Winston
Winston is a great pick for this mode for the same reason Symmetra is. Since Winston doesn’t really have to aim to start sending some serious volts through his enemies bodies, it’s easy to take advantage of enemies who are stuck floating around in the air. If enemies are jumping off a ledge or otherwise falling from a relatively high point on the map, Winston can jump on in and eliminate them before they see the ground again. Not only that, but Winston’s mobility is better than ever in Low Gravity.
Winston doesn’t have to be on the ground to use his Jump Pack, and there will be quite a few cases when the Jump Pack’s cool down has completed by the time you’re back on the ground. While other characters are in the midst of a dangerous no man’s land if they are too high in the air, Winston is one of the heroes that maintains a sense of mobility while airborne. Other heroes just have to wait until they slowly drift down but Winston’s ability to use his Jump Pack again gives him away to avoid danger even in mid-air.
