Examples from Injustice 1: Solomon Grundy, The Joker, Batman, Superman, Doomsday
Examples from Injustice 2: Scarecrow, Hellboy, The Joker, Bane, Batman, Darkseid, Deadshot
Aside from the joy of playing Injustice as a comics fan, I really really enjoyed that I could find something fun about playing each character. My top 2 are definitely Green Arrow and Batgirl, but it starts to get harder to choose from there. I definitely would also put Solomon Grundy and Batman up there too, I totally get why you have them in your top roster.
I never owned Injustice 2 and unfortunately didn't get to play enough of it to choose, let alone get to try all the awesome new characters they added. I'm sure a day will come at some point where I finally get a real upgraded gaming experience and can then get the sequel for myself!