Rezb1t and Latias, however they don't upload very often on YouTube (not sure about twitch).
Nikey was arguably the best sniper in Splatoon 1, but he doesn't seem to be using chargers in Splatoon 2.
ThatSRB2Dude (who is arguably the best player in the western competitive scene) doesn't use chargers in tournament, but he does use them while on YouTube and Twitch, and is still ridiculously good with them - he uploads regularly on YouTube, and often streams on Twitch as well - he's where I learned most of my charger skills.
Also Echo, who plays for team Extermination. I don't think he used YouTube or twitch, but if you can find some tournament footage of him on YouTube he's really good too.