Tobi wins without too much difficulty. He's shown to be pretty formidable without showing off his rinnegan (he has a ridiculous amount of chakra as well, enough to spam kamui), tbh the others would need to team up to actually land a hit on him.
Next I would say Madara or Itachi, but if the fight drags on I could see Madara outlasting Itachi.
Sasuke of course is the weak link.
Tobi is one of the last ones standing as he can go into his alternate dimension and chill out until someone emerges victorious from the remaining 3.
What is Madara in his prime? He said that he unlocked the Rinnegan like right before he died or something like that. Is that him in his prime? If so, it's him vs Tobi in the end. Really no matter what, I think it's him vs. Tobi in the end. As of today's date, I'll give it to Tobi since I don't know what Madara has to deal with intangibility. Anyone who can find a way to deal with that kinda starts beating the crap out of him with ease but I just don't know what Madara has yet. If it's revealed that he can do his own Kamui type thing or can move at 4th Hokage speeds, I give it to him.
Either Madara or Itachi...
Tobi WOULD but Itachi could easily figure out his weakness and can trap him in a genjutsu or figure a way around his technique... madara could also figure out tobi's ability and simply overwhelm him...itachi vs madara would be interesting because:
Itachi is smarter, very battle-smart, faster, and has a greater genjutsu ability with his eyes
Madara has tons of experience, and has a much more powerful eye in general. Madara also has a much greater amount of chakra and abilities at his disposel.
In the end, oh and sasuke just dies...either one of them could kill him without TOO much trouble.., i believe Madara wins majority like 7/10 but itachi is no push over and i definately do not put it past him to win