Your opinion may vary. But here is a thought provoking list from Information Week. I will give you the partial list but will let you get the top 5 from the original post.
12. The Morris worm
11. Google search rank
10. Apollo guidance system
9. Excel spreadsheet
8. Macintosh OS
7. Sabre system
6. Mosaic browser
My opinions are slightly different:
I would move Mosaic browser up
I would give the top spot to the original web server written by Tim B for its impact.
Discory Path: dogg.dot.us ->Slashdot > Information Week Article