Symmetra's fun to me. She's my most played hero, though I only play her on defense, and even then only on some maps. And I generally switch to Mei if the enemy starts approaching the final checkpoint.
You gotta set up your turrets on the flanks to make sure the enemies are funneled down one path to your damage dealers. You can use them kind of like an alarm system to know which path the flankers are taking. If you cluster them, make sure to leave enough space between them, and put them in strange places so that they take more time to find and take down, but don't make them so far apart that they can't all simultaneously attack a target coming through whatever path you're trying to trap. This works really well on Dorado (crashed train, side caves) and King's Row (back of the two-door building next to A, back of the statue on the roundabout).
Left click is the tool you use to let a enemy know "you fucked up." Use it when you see a low-HP hero trying to go it alone, or to help your teammates focus someone down. It works best against mechless D.Va, Genji, Hanzo, Junkrat, Widowmaker, and even Soldier 76 when you can get the drop on him. When it's charged, it can take down tanks fairly effectively, too. It loses to Reaper and McCree, and you're not likely to be in range to use it on Bastion or Pharah. You can also use it on healers if they're dumb enough to not be behind their teammates. Don't use it on the front lines.
Right click is really only good for three things: bypassing Reinhardt's shields, disrupting widowmakers, and stopping an advance down a straight path. It does a fuckton of damage, though, which makes it very discouraging thing to be hit by.