A bit morbid, but follow my logic:
When Heath Ledger died he left us with that great performance as the Joker. But what if he lived and ended up spending the next 10 years playing in spring-release chick flicks or going full-retard at some point?
There are actors or directors that I wish I could remember for the high point in their career. Had Kevin Spacey had tragically died just before he worked on Life of David Gale. Had Kevin Costner had a stroke just before day one of shooting on Waterworld. What if Cameron Crowe's private jet mysteriously disappeared over Kansas on the way from from the red carpet release of Almost Famous?
These are the movies and/or performances that I wish were my lasting impression:
5-Sean Connery - Hunt For Red October
4-Tom Cruise - Magnolia *
3-Al Pacino - Glengarry Glen Ross
2-Stephen Spielberg - Schindler's List *
1-Kevin Costner - Dances With Wolves
* apologies to Minority Report
EDIT: Not that people on the list didn't produce quality stuff afterward, but it's generally the top of the arc of diminishing returns.