Good question , First off Hal and Johns rings were the same. I believe a green lanterns power doesnt depend on the ring so much as the bearer ,and Hal is way way better than John . Kyles ring i feel is the best simply because yellow doesnt effect it like all the others and he dont ever have to charge it , but Hals ring protects from deadly injuries.
Hal Jordan is the only unique lantern cuz he's the only one that doesnt have an artistic background. kyle was,or is, depending on how far you're caught up as a reader, a commercial, and comic artist, while John Stewart was an architect. granted Ive seen more on kyle, and HAl as GL, and more than likely this applies to others too but its not really fair to count John out b-cuz he's the least popular