According to Frank Roche, of iFractal LLC, they are:
Steve Jobs introduces the Macintosh in 1984.
Dick Hardt’s Identity 2.0 presentation at OSCON 2005.
Guy Kawasaki’s Art of the Start speech at TiECon 2006.
Dr. Martin Luther King’s I Have a Dream speech in 1963.
Lawrence Lessig’s Free Culture talk at the 2002 Open Source Conference.
Malcolm Gladwell’s Blink presentation at SXSW 2005.
Tom Peters presents A Ham Sandwich in 1990.
Seth Godin talks about Marketing at Google in 2006.
Andy Kaufman sings along to Mighty Mouse on SNL in 1975.
See them all at Squidoo on Best Presentations Ever.