Heng Yen is a beast, his bleed, stun and crosshairs is awesome and can go behind Michelle, Carl/Harrison, Camilla and replace your decap, give some control and make a good bleed team with lacerators like Shiva.
Aarav is a monster! He can go on the same teams as Heng Yen and just rush constantly and wipe the floor with teams.
Lao Po is a tank, she feels much more tough to take out than Pete tbh. She is also great on attack as she can fully shut dpwn the enemy team, her daze, impair and attack down rush will make it so an enemy team cant do much of anything and with a tower in war to give her so more ap, if you have a huge bonus lead and weapon, she is rushing turn 3 like everyone else and love using her in war behind Camilla.
Mercer is the meta/latest and greatest right now, stun lead skill is a pain, he does a lot of damage and stuns in his rush and his weapon is a great chance to stun gun which is amazing even if you have stun guns or focus stun, great chance is 50%
Princess is good and would be good behind Pete, Priya or Mercer. She can end up doing a stupid amount of damage to a line, with her skill and weapin if set up right, she will kill any line of toons. Normalize skill is amazing as it is a counter to those annoying payback amd bide toons.
It kinda depends, what i would do is pick the toon that you will use the most and build the most teams with. Can you build a team behind Mercer or Lao Po or Heng Yen, etc? How many teams can you use Mercer, Lao Po, Heng Yen, etc on? What do you need, do you need a control toon and lead like Lao Po or do you need raw damage like Aarav or Princess or maybe you want to have a decap toon that you can make a bleed team with like Heng Yen? It really depends on what other toons you will put with them/where you can put them into other teams.