Setting village hidden in the rain
This is before Tobi gets the rinnegan
In this fight to see who should rule the world who will win?
Hasn't this been done before?
Anyway, in a straight fight my money is on it being a draw or Pain winning. It's a bit of problem to put Tobi in a fight against anyone, because against most people it will be a draw.
Tobi isn't winning this, as pretty much his only strategy is to warp his opponent away. With 6 paths coming at him at once (plus summons), I really don't see it happening. But then if Tobi decides to simply camp, Pain has no way of landing a hit on him except with a surprise attack (which would be doubtful).
The interesting thing about naruto is that there are many battles in which either person can win. In fact this battle is all about who prepares the best.
Tobi: As the leader of the Akatsuki it would be no surprise that he would know how nagato's powers work, and so he will be checking out all of the high areas for Nagato's location. With his teleportation abilities as well as his sharingun and intangibility he will be able to detect patterns and be very cautious.
Nagato: has the rinnegan, and with the six paths of pain will be making alot of summons to keep tobi on his toes, along with the pain that spams out missiles all the time, he will try to overcome the intangibility time limit of tobi (see Konan fight for reference). Nagato will also likely keep Tendo pain with him in order to push or pull tobi. In addition Minato was able to exploit the fact that tobi has to materialize in order to attack, and so there is no reason why six paths of pain would not try the same trick.
Objectively there are scenarios in which both could win, though it is far more likely that Pain will win, due to the sheer number of tactics he can use.
Therefore, Pain wins IMO 7/10 times.
@MisterShin: To be honest I don't see optical genjutsu working on rinnegan, since it's been proven to be an evolved form of the EMS.
Itachi is a beast with his susano, but I doubt he'd have the endurance to keep it going for too long. Also even his susano has no way to counter Chibaku tensei.
And just because someone is taking orders from someone doesn't mean they're weaker; Tobi brainwashed Nagato, that's all. Btw, even if he does have access to Danzo's comrade (shouldn't really count as it's meant to just be Tobi), it wouldn't make much of a difference.