MIT researchers did a study to rate the most popular Americans of all time.  They came to the conclusion based on information such things as Wiki page visit stats and search engine numbers.
Here are the Top 10:

1) Martin Luther King Jr.

2) Elvis Presley

3) Walt Disney

4) Benjamin Franklin

5) Edgar Allan Poe

6) Bill Gates

7) George Washington

8) Thomas Edison

9) Johnny Depp

10) Abraham Lincoln