Dick grayson, I think it would be cool if supes helped out the flying graysons on their tour to smallville or metropolis and Dick was there...
it would be the closest and most legitimate reference to bats they are likely to get in smallville.
that or perhaps john stewart should be a drill seargant or an officer who's served in the war and is taking on clark who thinks the best way to help others is to serve in the military.
diana would be too easy, and i only think he should meet diana last out of the justice league members and they should all meet her together.
however an obvious choice would be hawkman. since we have already met league members that are more skilled than clark in the natural habitats (bart on land and arthur in the water), perhaps the next thing is to find one who is better (either more agile or faster) than clark in the air.