This is something I came across a long time ago looking through a friend's comic books, and I found it again recently online here (from Wizard Magazine #70). This guy copied the article out seemingly word for word, even including the same pictures. If you don't want to read through the whole thing, the list is as follows:
10. The Hulk
9. Firestorm
8. Professor X (pre-Onslaught)
7. Spawn
6. Doctor Strange
5. The Flash
4. Wonder Woman
3. Superman
2. Thor
1. Silver Surfer
Obviously, supervillains and all-powerful beings like Galactus are not included. If they were, most of these heros wouldn't be listed at all.
Thoughts?