Which ones?
Alec Guiness was nominated for an Oscar for best supporting actor, but lets be honest, its because he was old and hadn't been in something in a while.
Harrison Ford always delivered fun performances but nothing major way.
All three leads in TPM (Neeson, McGregor and Portman) had delivered MUCH better performances in previous movies, and continued to do so afterwards.
Christopher Lee was great but didn't really have enough screen time to be much more than cameos.
Samuel Jackson delivers his most wooden performances ever, and its hard for me to imagine him delivering anything woodenly. Even his "whats in your wallet" commercials show better acting chops. That man could show up at your door to deliver pizza and show more energy than he had in the prequels.
Mark Hamill in "A NEW HOPE"
Alec Guinness in "A NEW HOPE" (I don't believe that the reason why he got that Oscar Nomination was because he was old and hadn't been in something in a while)
Harrison Ford in "A NEW HOPE"
Mark Hamill in "THE EMPIRE STRIKES BACK"
James Earl Jones in "THE EMPIRE STRIKES BACK"
Ian McDiarmid in "RETURN OF THE JEDI"
Billy Dee Williams in "RETURN OF THE JEDI"
Harrison Ford in "THE FORCE AWAKENS"
Carrie Fisher in "THE FORCE AWAKENS"
Daisy Ridley in "THE FORCE AWAKENS"
Adam Driver in "THE FORCE AWAKENS"
Domhnall Gleeson in "THE FORCE AWAKENS"
Andy Serkis in "THE LAST JEDI"
Felicity Jones in "ROGUE ONE"
Ben Mendelsohn in "ROGUE ONE"
Mads Mikkelsen in "ROGUE ONE"
Emilia Clarke in "SOLO"
Donald Glover in "SOLO"
Woody Harrelson in "SOLO"