Keep in mind these pickrates are for quickplay and competitive. And of course, high pickrate does not mean a high winrate.
Top 5:
As expected, Batman is the most popular pick globally. However he also has the lowest win rate in the game. This is expected as he's popular with newer players, which natrually lowers his win rate.
Nightwing is a bit of surprise to me as he was nerfed a few seasons back, but his kit such as his double jump, grapple, batons and his Flying Graysons passive make him an excellent counter to almost any character. Bonus, he's especially good at diving backline supports.
Superman is the best tank in the game, so his high pickrate is expected, especially with the newly introduced mandatory tank role. He's the only tank with three passives, those being Kryptonian Might and House of El and Solar Absorption. Top it off with great utility from his ultimate, and Superman becomes almost broken.
Joker was low tier these past few seasons, however recently Joker's Venom Spray had its cooldown reduced from 14 to 8 seconds and they added an anti-heal effect to it. This alone made him top tier for competitive play. Alongside that, Laughing Gas AOE range was buffed so that it doubles its range if it stuns 2 or more players. With that you've got a CC nightmare to deal with.
Harley Quinn sadly is in the same place as Batman. Her popularity drives her pick rate high, but that does not reflect in her win rate. Her movement, DPS and passives are just average. Why play Harley when you can play Red Hood, he does eveything she does but better.