I'm chiming in a bit late here, but I was randomly struck with a thought regarding the Praetorian Praefecture. I suppose it could be argued that the social/political status of the Praetorian Praefect may have been fairly dependent upon the Princeps himself. The accumulation of power occurs most earnestly under the usual suspects (ie Sutorius Macro under Gaius, Burrus with Nero, Perennis and Cleander with Commodus, etc.)
and the clear case of Sejanus with Tiberius.
A case could be argued that the Praefectus Aegypti was a very close rival for the top Equestrian post, and its difficult to argue with provincial appointments such as Britannia, Pannonia and Syria with command of multiple legions. However, when we consider that Vespasian actually made Titus (who had been clearly marked as Caesar and heir from the very beginning of Vespasian's victory) the Praetorian Prefect, the position's importance and standing is quite clearly illustrated. That does not necessarily stand as an absolute since Titus still outranked any other position by virtue of his status as "Caesar" , but I suppose it lends additional support to the notion that the Praetorian Praefect was essentially the top ranking appointment in the Principate.