Are you familiar with the page in which Juubito says that he couldn't use kamui? Whatever,this is your thread so I'll answer this according to OP conditions: Obito Uchiha wins this with or without Kamui.
Without intelligence on the kamui, I don't see how Hashirama can prevent getting warped. He's too lethargic in his movements. With intelligence, wood clones would definitely be a good counter.
Obito with Kamui is a very diffcult opponent to anyone WITHOUT:
1. Knowledge of how it works.
2. access to that dimension (like naruto had when kakashi Kamui'd his rasengan)
3. Speed like Minato's
So, as much as I think Hashirama could beat just about anyone barring juubi transformations, Obito could actually win without transforming.