Off the top of my head,
John Delion, the guy who stole his fortune
The Clock King
Camarogue, a sneak thief with some kind of chameleon-like invisibility suit
Slingshot an assassin who used David and Goliath style slings to kill people
The Rainbow Archer
Bullseye, an acrobatic clown
Milo Armitage, Connor Hawke's criminal stepfather
The Bamboo Monkey, yet another evil archer, although from what I've seen this guy relies on regular pointy arrows and a lot of skill.
Chein Na Wei and Hackett, the villains of the recent Green Arrow: Year One mini series. Only appeared once but they had a role in turning him into a hero.
Shado, as I understand it she's more of Catwoman like ally/enemy but I believe they started out as opponents. Mother of GA's youngest son
Brian Nudocerdo star city's corrupt Police commissioner
Either Professor Millions or Professor Merlin. I confess I don't really remember this character's name but in the golden age he'd become embroiled in crimes while trying to raise money for his college
Honorable Mention:
Cheshire, genocidal assassin and mother of Ollie's granddaughter Lian. I don't think she's ever appeared in a GA comic but she's a big part of Arsenal's backstory.
and I think Catman's got a grudge against him