The Question. And not just because he's a fan favourite. For one, he actually brings something new to the table. He's an investigative thinker; a talent Ollie has yet to show a propensity for. If introduced in Season 1, he'd be the perfect guy to clue Ollie in on the conspiracy going on behind the scenes and connect the dots between all those people in Ollie's hit list. Likewise, his look fits perfectly well in the show's setting. A guy in a suit, trench coat and fedora? Classic CW.
Also, maybe Renee Montoya (though not as The Question). At the moment, one of the things lacking on the show is for Green Arrow to have an ally in the force. Though, I wouldn't be surprised if Laurel's father does eventually fill that role.