I think most of you are confusing occupations, rankings and general names.

Sannin = Collective reference to Orochimaru, Jiraiya and Tsunade, due to their achievements. Its not a rank.

ANBU = Comprised of jounins and chunnins. Its more of an occupation as opposed to a rank.

Hence, I think the ranking order should be:

(Ho)Kage
Jounin
-> Special Jounin (Not much information on these, but I'm assuming they're a cut below normal jounins)
Chuunin
Gennin
->Student (doesnt really count)

Also consider that the ranks are more of a guideline as to the capabilities of a particular ninja as opposed to a hard and fast rule.

Hence, if we add the information we would get this: The Sannin are presumeably all jounin rank. They just happen to be far stronger than the average jounin - indeed you could argue that theyre probably stronger than some Kages.


******* - MINOR SPOILER - *******


Itachi is apparently a chunnin. However that doesnt change the fact he is the strongest character we've encountered so far (In the anime).


******* - END OF SPOILER - *******

So, if I were to put together a list with both ranks and people/occupations, on the basis of their power it would look something like:

Kage | -> Sannin, Itachi
Jounin
Special Jounin | -> ANBU
Chuunin
Gennin

Note: the "| ->" indicates that the following is NOT a rank.