I think impeachment and removal from office should be reserved for the most serious of offenses. Had Nixon not resigned, impeachment and removal from office would have been deserved. Clinton certainly didn’t deserve to be impeached. He wasn’t exactly husband-of-the-year, but that wasn’t an impeachable offense.
Andrew Johnson is a bit trickier because he blatantly violated an act of Congress and almost dared them to impeach him. Congress, however, deserves some blame in the matter as they almost dared him to violate the Tenure of Office Act and were dying to bring Johnson down. Still, the President should be above such petty gamesmanship and Johnson broke the law in a much more serious way than Clinton did, so he deserved impeachment.