The simple answer is, none of them - assuming of course we're talking about events that took place in a prior administration. The framers were wise to put a mechanism in place to indict, convict and remove sitting presidents from office. This controversy was no "secret" to the 2006 session of congress. Nancy Pelosi could have held hearing, and if the facts and evidence supported it, she could have moved for the impeachment of President Bush. But she didn't, and the reason she didn't was precisely because the politics of that decision would have spelled disaster for the newly minted Democrat majority. You see, this is just as much a matter of politics as it is a matter of law.
The moment we head down this road of investigating administrations once they leave office, is the moment we place the country in a death spiral, from which there may not be a recovery.
When Clinton left office, many in the grass-roots conservative movement wanted all manner of investigation into the Clinton administration. Thankfully, cooler heads prevailed, and we didn't see that transpire. Ford was another man that put country ahead of his own personal political viability. He pardoned Nixon, ending any chances of him being elected to the office he held but he saved the country from 24 months of turmoil, and he kept a horrible precedent from being set.
Make no mistake, irrespective of the liberal protestations about this being an investigation to "uphold the rule of law", this is nothing but blatant and overt appeasement of Barack Obama's rabid leftists base - nothing more, nothing less. Bush (and the GOP Congressional leadership) was right not to indulge such folly in 2001. Obama, once again proving that he's not half the man of Bush, to say nothing of Ford, can't contain his glee in this ex post facto witch-hunt.
I disagree. What you are saying is that a President can direct his minions to break the law in any way imaginable, but this criminality can only be addressed by means of impeachment, something obviously impossible if his party controls Congress.
Then even if the people revolt and hand control of Congress and the Presidency back to the other party, nobody in the previous administration can be held responsible for criminal acts.
I agree with your characterization of this investigation as politically motivated, I just am worried by the idea that all members of an administration acquire a get out of jail free card on leaving office.
My personal opinion is that the Constitution says nothing about government officials being above the law and not liable to indictment, conviction and punishment for criminal acts.