Dude, Where's My Ranch? - S14-E18
Plot hole: Lisa says Luke was her 1st crush. What about Nelson, Corey, Mr. Bergstrom, Thelonius and the boy that works at the library in 'Bart's Girlfriend'?
Suggested correction: She didn't have much of a crush on Mr Bergstrom, she mainly admired him because he was kind. However you are correct about the others.