Batman's co-creator,Bill Finger,also had a hand in the creation of the Green Lanterns.

Sinestro with Star Sapphire as a close second because I like hero/villian love stories.

I went with Sinestro, mainly because unlike most of DC's mirror villains, he's a little more believable, and a great cautionary tale for our hero.

I'd also like to throw some love to Solomon Grundy, just because there is another Green Lantern besides Hal Jordan. I always preferred Alan Scott to Hal Jordan. While he had less of a personaility, i found the GA GL's ring flaw (wood), so much more believable than the colour yellow. I know, a 7 year old's logic, but there you go.

The Shark!

viewtopic.php?f=1&t=52888

I'll go with The Weaponers of Qward, even though I haven't read about them in years.
When DC got rid of alternate earths, they even retconned The Crime Syndicate to be Quardians, but they didn't really do much with them.