Main: Catwoman.
I wanted to main a lady since I first saw Harley, and combed their ranks even while piloting the first few weeks with Deathstroke, and this girl is the one for me (see my insane Cyber Cat Theory thread for more elaborate delusion).
She's fast, she's consistent, her trait reminds me of Tifa's Limit Break from FF7, and she can cover an entire screen with one bar.
That's my new age Cyrax, my psychotic feral princess, my boo boo kitty fuck.
Secondary: Deathstroke.
I've never seen more infuriating and entertaining zoning in a character than 'Stroke. The early-game-life satisfaction of smothering enemies in ammunition cemented a place for all days in my roster for Slade Wilson. I firmly believe he'll be a consistent tier-skippity-placement-uncertain warrior throughout IGAU's life, and the keepaway damage he can do to big characters who don't know the MU well enough to escape the bullets...it's quite humorous.
This is the new Sub-Zero, the defensive fortress, The Most Interesting Mercenary In The World.
Tertiary Trio: Doomsday, Aquaman, ???.
The character whore in me knows that it's always best to learn your enemy so as to better defeat him.
Plus, it never hurts to know the basics and beyond of two (three) of the outright best characters in the game.
If you can fit the moves in your head, remember them and don't end up dead. Play to win.