With Karthikeyan I mostly think of the 2005 Chinese Grand Prix where he somehow managed to crash on the backstraight. If I was thinking postively, I would think of the 2011 Indian Grand Prix, or the 2005 United States Grand Prix which were the high points of his Formula One career.
When I think Michael Schumacher, I think of the 2006 Braziliam Grand Prix where his climb through the field was simply amazing.