My criteria is how much they get complained about online and in communities 
For me, I’ve noticed Karax, Stukov, and Swann
I’d appreciate if Karax got buffs but TBH, I’m fine with the way he is now.
I miss Stukov’s pre-nerf bunkers, but I like the changes to mech. We’re polarized, as some say he’s ruined. Others say he’s still awesome
Swann just takes a while to ramp up.
In fact, I tend to prefer defense, support-oriented COs, or if they take longer to ramp up anyways