In Nixon's case the biggest thing was the cover-up and the suggestion that the IRS be weaponized against political rivals.
I didn't see any major scandals during the Carter administration, just general incompetence.
Reagan was definitely the Iran/contra scandal
HW Bush didn't have much at all that I am aware of.
Clinton sold weapons technology to China giving them the ability to hit our mainland, the Lorale scandal, The Rose Law firm scandal, Sandy Berger stealing classified records, and lying to a grand jury under oath,
Bush I don't remember having any legitimate scandals
Obama had Fast and Furious, Holder being held in contempt of congress, Spying on journalists, farming metadata of everyone in the country, Bergdahl, weaponizing the IRS, the EPA, NOAA, and other departments, The Deepwater Horizon, the Colorado river, the VA scandal, And the kickbacks to Solyndra and other firms.