It seems like a tough call, in a world full of deformed psychokillers, mad scientists, and giant purple mind-controlling starfish from outer space. But I know who gets my vote:
Gypo-Bax.
Don't beat yourself up if you don't know who Gypo-Bax is. After all, he never got a Mego, never showed up in a crossover cameo, never even got mentioned in the DC Encyclopedia. In fact, we've never seen Gypo-Bax, we have no background information about Who He Is and How He Came To Be, and we have no knowledge of anything he has ever done.
We just know one thing about Gypo-Bax:

He's as evil as Sinestro.

Or even more evil.

Remember, the whole Green Lantern story is kind of a high school sitcom. Hal Jordan is an annoyingly macho brain-dead jock-type and Sinestro is his Reggie Von Mantle, who participates in annual Evil Popularity Contests. I mean, look at Sinestro:

Poor Sinestro; Gypo-Bax kicks his butt two years in a row.
Yes, as ridiculous as the Qwardian evil popularity contest was the first time they used it in a Green Lantern story, they used it again, compounding the absurdity. But it leaves a gaping hole in the Green Lantern tapestry...
Who is Gypo-Bax? What does he do that's so evil? Do Qwardian children see Gypo-Bax's face on their box of Meanies, the breakfast of evil champions? Does he dislike Hostess Fruit Pies with Real Fruit Filling? Is he the former DMV director on Oa?
Was/is Gypo-Bax actually more evil than Sinestro or just a lot more successful at it? And now that Sinestro is like Bill Gates, all powerful and able to make people work for him at Microsoft/The Sinestro Corps, does he take any revenge on Gypo-Bax? Does he send evil bug-people with tiny rings to eat G-B from the inside out? Does an aged Gypo-Bax, now in a wheelchair at the Old Evil Folks Home, get a invitation to the Corps, exult for a minute in his regained vitality and newfound power, only to have Sinestro show up and shout "PSYCH!" and snatch the ring back? Now THAT would be evil.
Come on, Geoff Johns. How can you pass up this opportunity?