I haven't read the other posts yet, so that they wouldn't influence my choices. Answers will probably be pretty biased based on my age and preferences, and may not reflect their best performance, just their most famous one.
A) Tom Hanks: Forrest Gump
B) Mel Gibson: Braveheart
C) Arnold Schwarzenegger: Terminator
D) Sylvester Stallone: Don't know his movies
E) Bruce Willis: The Sixth Sense
F) Tom Cruise: Top Gun
G) Will Smith: Men in Black
H) Robert Patrick (other than the T-1000): Don't know who this is
I) Harrison Ford: Indiana Jones (I chose this over Star Wars because it was more of a leading role)
J) Richard Gere: Some sappy romance, I don't watch those
K) Robin Williams: Mrs. Doubtfire
L) Robert DeNiro: Analyze This
M) Jim Carrey: Ace Ventura
N) Samuel Jackson: Jurassic Park
O) Jack Nicholson: The Shining
P) Morgan Freeman: Driving Miss Daisy
Q) Danny Glover: Haven't really seen many of his movies, not sure
R) Joe Pesci: Don't watch his movies
S) Eddie Murphy: The Nutty Professor
Bonus Question: Which of the 3 actors above are most famous for their overall work/roles?
ETA: I'm impressed that you guys actually named the characters from the films, no way I could come close to remembering that, outside of Forrest Gump, Ace Ventura, and Indiana Jones.