Two categories here, met and seen in person. I lived and worked in L.A. many years ago so you would see celebrities from time to time.
Met:
Richard Kiel (Jaws in Bond movies), chatted with him in an airport while waiting to board.
Mohamed Ali, twice at fund raising dinners - brief conversations when he visited every table to shake hands with everyone there - two different times - couple of years apart.
Seen:
Jack Lemon, on Hollywood Blvd.
Dinah Shore, at a camera shop
Sidney Poitier, at a movie theater, with his wife.
Bette Midler, at an iconic hamburger place, with her husband.
Sylvester Stallone, sat next to his entire family, on a patio at a restaurant in Palm Springs
John Spencer, actor on L.A. Law, at a restaurant
Howard Hessman, WKRP Cincinnati sitcom actor, at a movie theater
Don Knotts, at a movie theater
Carroll O'Connor, on the street in west L.A.
Sterling Hayden, at J.C. Penney
Royal Dano, at J.C.Penney
Those are the ones I can recall now, probably others as I lived in L.A. for 17 years.