Greatest of All Time is very tough to measure because tastes and preferences change.
I watched the Departed again last night and still enjoyed the shit out of it.
Others that come spontaneously to mind that I really enjoyed.
Kramer vs Kramer
The King's Speech
Dangerous Liaisons
Last of the Mohicans
and fuck it if I won't always watch Snatch when it's on (yes - I know it's not the GOAT)