(all characters are at least level 10):
Catwoman - A good card unlocked in the first stage, after tutorial, and is worth keeping your line-up early on due to the fact that you get quite a few upgrade/support cards for her in another stage and she can do a one-hit kill special attack. Not worth having later on due to low damage rate.
Deathstroke - A good starter card with a high damage rate and effective basic attacks.
The Flash - The best starter card and well worth keeping in your line-up. Great damage rate and very effective in battles.
Green Arrow (from Silver card pack) - Effective basic attacks accompanied with his freeze special attack that allows you to do bonus damage. Arrow is certainly one you want to keep your eye on.
Green Lantern - All his strength is in his one-hit kill special attacks. Aside from that, I wouldn't really recommend him.
Harley Quinn - Like Green Lantern, All her strength is in special attacks. Aside from that, She is worthless do to low damage and even lower health stats.
Nightwing - Low damage rate but makes up for it with speedy basic attacks. Special attacks are not that effective. Not worth having in my opinion.
Sinestro - Slightly lower stats than Green Lantern. Again, Poor basic attacks but makes up for it with strong special attacks. Wouldn't really recommend.
Solomon Grundy - Very low damage rate but incredibly high health with a regeneration bonus when near death. Great as a tank but eventually his use runs out.
Superman (from Gold card pack) - Is fantastic. Incredibly high damage and health that is great to have early on, Mind you I got it from syncing someone's WB ID to the game and unlocking him. A must have.