Sinestro would be the obvious choice if Hal was in fact the Green Lantern to have his first film. The choice of Hugo Weaving is one I'm fond of, and can see him playing the role greatly.
The way I see it happening, is based on how he became who is now from the comics. He was assigned to instruct Hal when he first joined the Corps, but after a rebillion from the natives of his planet Korugar, he was reported by Jordan himself leading Sinestro to get his ring taken away from him by the Guardians. He was known as one the of the greatest Lanterns on his time, but now was fueled by revenge on Hal for what he had done to him.
After is when he adapts his yellow power ring, which gave him more of an upperhand to the other Lanterns considering their rings can't effect anything yellow. But it's skilled Lanterns (like Hal), who've found ways around it in order to defeat him.
Seeing the two clash on screen like that would be great, if done right. Not to mention the amount of CGI that'd be required for it.
