Here's my list:

Ted Kennedy - on a flight to Zurich giving his aide a list of christmas presents he was to buy once in Zurich.

Ron Howard - JFK-LAX

The guy who played 'Q' on Star Trek Next Generation on a flight into Washington (who I had to show how to operate the inflight phone).

Arthur Ashe - on a flight to DCA

Katherine Helmond - on a flight from SXM (also on the earlier connector little puddle jumper from St. Barth's)

There are others I can't remember, but my all-time favorite was on Qantas (OneWorld kind of counts). This December headed LAX-SYD (a couple of weeks before the Australian Open) Venus Williams and entourage sat in the entire row 4 in F and while I was standing in the aisle during boarding (I was in row 2) a blond woman bops me on the head with her carry-on and when she turned to apologize it was Monica Seles. Interestingly they didn't acknowledge each other until we landed and were deplaning when they made some tennis small talk.