Prolly the best cards in the current meta are Arkham Origins Deathstroke (8% per promotion damage bonus/block by passing auto critical special 1), Raven Prime (people often complain about her twice per match health switch saying that it's broken and she's a really strong character if you team her up with Wonder Woman 600 (30% power generation and damage boost for female characters) and Animated Harley Quinn (20% power generation and damage boost), Animated Batman Beyond (if you have a complete set of alliance support cards, his special 2 can inflict well over 150k+ critical damage if he's teamed up with Animated Harley Quinn and Batman Prime).
If you're trying to make a team, make sure to have atleast one anchor (the guys above are okay), one support (Animated Harley Quinn/ Regime Killer Frost/ Regime Green Lantern/ WW600), and one wall (preferrably God Fall Superman with Static Shock's Charged Disc and his signature Krptonian Battlesuit for the reflect and health regeneration as he's centered around reflecting damage).
I'm sure there are plenty of other cards out there that are decent like Man of Steel Superman, Arkham Origins Batman, and the newly released MKX Scorpion.
Oh, and a character with a multi hit combo ender like Nightwing 52, Arkham Origns Deathstroke, and Hawgirl Prime are broken equipped with the Ra's Al Ghuls Scimitar.
