Well Lets See.
Naruto and Konohamaru are currently the only people that cannot, without aid of clone, or additional chakra source, make a normal Rasengan with one hand.
Naruto however, can do it easily in Kyuubi Mode, and yet has still been shown doing it even in Sage Mode, still requiring clones to make Rasengan and Rasen Shuriken.
Although he has more forms of Rasengan than all of the others put together (That we have seen). With that said, he has to be placed as number one, despite him not being able to make a one handed Rasengan.
Minato would be second, being the original maker of the technique and using it in conjunction with FTG just makes it reckless.
Jiraya, having more chakra than Kakashi, would naturally be the best person to go third. That along with Sage Mode enhanced speed and power gives him the advantage.
Kakashi's use of Sharingan with the technique, )If he even used it which he doesnt since he has chidori, )and the speed that he has built up with the use of chidori, enables him to quickly and more accurately than any of the above strike a target with rasengan to cause the most damage
Konohamaru doesnt have anything special about him right now. He still needs a clone to make Rasengan like Naruto.