Jonathan and Lionel
From Smallville Wiki
"Our fathers were a lot alike. Both smart, strong, and died prematurely." Lex to Clark - Descent
The animosity between Jonathan Kent and Lionel Luthor started before the show began, and continued until Jonathan's death in Season 5.
Jonathan hated Lionel for years and became worried when Clark began a friendship with Lex. Jonathan thought that Lex was like his father, a manipulative and evil man. Despite his wife's and son's faith in the Luthor men, Jonathan remained distrusting of them, and was uncomfortable every time either of them associated with either Lionel or Lex. When Martha got a job to be Lionel Luthor's personal assistant, it caused friction between Jonathan and his wife.
The feud ended the night Jonathan died, when he attacked Lionel for apparently threatening to expose Clark's secret. Lionel claimed that he was misunderstood; he wanted, rather, to help Clark keep his secret. Despite this, Jonathan made it clear to Lionel that he warned him to stay away from his family, and that he would not let him destroy it.
