THe executive, as always. Because the executive branch as the army and the police.
I'd agree that the executive would be the most likely, but I think mainly because the power there is largely consolidated in one person and, thus, it doesn't take a 'cabal' to move in that direction. Certainly, having the army helps i you really want to go tyrannical.
That being said, since some of these things are thinly defined in the constitution (and would be even less defined if not working on checks and balanced), it's possible that any of them could go that way.
For instance:
- The judicial branch might decide that they have the 'final say' on ALL laws, set themselves up as a veto body (sort of a 3rd house of Congress), and then effectively dictate what the laws have to say to get past them.
- The legislature might realize that they have the power to effectively 'buy votes' by their control of expenditures and, thereby, effectively guarantee that they'll be in power for life. Ok, so they've already done that one, but it took them a while to get so blatant at it.