well, even though I love Orochimaru, I'd say his better claim to fame is having the most amount of Jutsu out of that list, or any Shinobi.
If it's for power, then naturally you've got to think that Naruto will at some point be the strongest, almost without a doubt, and with Sasuke not very far behind.
Madara isn't on the list, but even if he was, I know people could contest his power as well.
Pein is very strong also. Judging on what has been shown so far, I have to think that either Madara, not in the poll or Pein.
It just seems to make sense that at this point in the series one of the known leader/founders of the Akatsuke, the main antagonist group thus far, would be the strongest.
At the same time, I know everyone can make very good arguments for Itachi.
Really, it almost comes down to opinion in many cases, but a good Madara argument will probably trump an Itachi one. Maybe it's a bit like rock paper scissors
as for the 4th......well like Rei Uchiha said, we don't really know much of anything about the guy aside from the fact that everyone speaks of him in the highest reguard and like a legend! I just can't go out on a limb and think he's the strongest, and anyway, he's dead, I saw his casket! lol
all that being said, Orochimaru wins in the end. 100 years in the future, he's still going to be alive and still learning jutsu hahaha
(unless someone seals him before Sasuke dies or loses his will to him)