Last I saw of Loki, he had become the God of Stories. He had gotten power over stories, and in a comic, that gives you the power of a narrator and all that entails. Loki wouldn't even fight Sinestro, he'd wrap him up and throw him away. He's the one who saved the essence of the ten realms from the last Multiverse and brought them into this one.
If we're talking about classic adult evil Loki and Sinestro? I'm not as well versed in Sinestro but Loki's feats off the top of my head are superior (?) but like I said, I don't know his best outside of mustache twirling days. He has beat the f*ck out of Mongul and if he's anything like Hal Jordan, he probably shits on every other Lanternverse character.