Here is my list (all conditions considered)
1. Sammy Wanjiru- 2008 Gold/Olympic Record in Beijing 2:06:32
2. Haile Gebrselassie- 2008 Berlin Marathon- 2:03:59 World Best
3. Paul Tergat- 2003 Berlin Marathon- 2:04:55 World Best
4. Jaouad Gharib- 2008 Beijing Summer Olympics- 2:07:16
5. Duncan Kibet- 2009 Rotterdam Champion 2009- 2:04:27
6. James Kwambai- 2009 Rotterdam 2:04:27
7.Khalid Kannouchi- 2002 London World Best 2:05:38
8. Martin Lel- 2008 London Marathon Champion- 2:05:15
9t. Cosmas Ndeti- 1994 Boston Marathon- 2:07:15
9t. Robert Cheriyout- 2006 Boston Marathon- 2:07:14
Am I forgetting any great performances?