The most egregious example of foreign involvement in elections was the Iranian Ayatollah Khomeini’s support for Reagan’s election over Carter.
According to The Washington Report on Middle East Affairs:
… American journalists. Bob Woodward and Walter Pincus have reported in the Washington Post and Alfonso Chardy in the Miami Herald that three Reagan campaign aides met in a Washington DC hotel in early October, 1980, with a self-described “Iranian exile” who offered, on behalf of the Iranian government, to release the hostages to Reagan, not Carter, in order to ensure Carter’s defeat in the November 4, 1980 election.
… ‘We don’t have to worry about an October surprise’ a jubilant staffer at the [Reagan] campaign’s operations center (told Honegger). ‘Dick’s cut a deal.'”
“Dick” was Richard Allen, and the deal apparently was a promise of arms in return for a delay by Tehran in releasing the hostages. A few days after the conversation Honegger describes, another Reagan campaign official, future CIA director William Casey, was sufficiently confident to tell journalist Roland Perry on October 30 that if something happened to give Carter the election, “it won’t be the hostages.”
In return for the hostage release after his inauguration, Reagan reportedly promised Iran arms that would be provided by Israel, which also wanted to curry favor with the new Reagan administration, and of course Reagan also rewarded Iran with the infamous Iran-Contra deal, providing Iran with missiles it should not have gotten legally.
In Reagan’s case there are many allegations that his campaign conspired with the Iranian government not to release the hostages before the election so as to increase the chances of Reagan’s election. In Trump’s case there is so far no indication that Trump conspired with Putin to defeat Hillary. Whatever Putin did, he seems to have done it for his own reasons, not because he was requested to do so by Trump, making Putin’s actions much less egregious than Iran’s on behalf of Reagan. Carter was too decent to put the US through the acrimonious sparring of the legality of the election like Obama and Hillary have done. Gore also accepted his questionable defeat from the Supreme Court without throwing the country into a constitutional crisis. Today’s Democrats are seeking a constitutional crisis by trying to use the intelligence community to invalidate Trump’s election.