[Rhûn]

If you're focusing purely on ranged combat, then it's Coran. Though it's a close thing.
I play BGTutu in which Kivan is an Archer, allowing him Grandmastery in the longbow. However, I also use multiclass Grandmastery patch, so Coran would be able to get it too. In my view, they're both excellent archers, the two best in the game, with Kivan slightly below Coran in hit percentage (though Kivan always seems to get more kills...).
It really doesn't matter which you use. Just depends on your personal taste (i.e. which character you like more) and party balance. If you need an archer who can hold his own and is more than adequate in melee, pick Kivan. If you are lacking a thief or are a fan of backstabbing, pick Coran.

(Oh yeah...gauntlets of ogre power make Coran a very good melee too, but again it depends on your party. I usually take both Kivan and Coran, give Kivan the bracers of archery, Coran the gauntlets of ogre power, and that makes them both supremely powerful characters)