Which Obito are we talking about? The one we've actually seen in the manga, with limited, low scale Mokuton, and only several Rinnegan techniques, all of which are for support, none for offense? Or the fan-fiction version that has Mokuton on Hashirama and Madara's level, and can use all the Rinnegan abilities (despite only having one eye) on par with Nagato?
If we're talking about the former, Kakashi has a legitimate shot. His physical capabilities are on par with Obito's, if not outright superior (speed, reflexes, Taijutsu). Their ninjutsu is on par, albeit Kakashi has more versatility. Hitting Obito will be an issue due to phasing, but considering Kakashi can just warp a shadow clone to camp in the Kamui dimension, that option can be potentially nullified. This is, of course, assuming Obito does not have access to the Bijuu that were handed to him on a silver platter (i.e. the accumulated effort of many Akatsuki members) nor the Edo Tensei Jinchuuriki that Kabuto gifted him.
Oh, and for people who continuously refer to "Kakashi getting help," . . . Kakashi started fighting an entire day before Obito even entered the war. He joined Naruto and Bee on Obito's battlefield after a full day of fighting Edo Tensei swordsmen. He was further worn down fighting V2 Jinchuuriki that Kabuto gave to Obito. He then wasted even more chakra directly fighting the ten tails, not Obito himself. Him having his chakra replenished only makes up for the energy he spent on efforts that were not a legitimate part of Obito's power. Obviously, Obito has more chakra than Kakashi but Kakashi's chakra replenishments are nowhere near as "unfair" as people are implying they were. In a one-on-one fight, it probably wouldn't even come down to who has more stamina.
If we're talking about the latter, Obito easily takes it with village sized Shinra Tenseis and a mile-long Mokuton forest.