for my eyes, scorsese has done so much great movies, so it's hard to vote this one even if taxi driver can't be beaten because of the outstanding performance of robert de niro. and there are still movies i haven't seen yet.
1.Taxi Driver
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2.Good Fellas
3.Aviator
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4.The Age of Innocence
5.The Departed
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6.Cape Fear
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7.Raging Bull
8.Gangs of New York
9.Bringing Out the Dead
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10.Mean Streets
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11.Casino
12.The Color of Money
by the way, i don't think that scorsese is overrated. if you don't like his style, ok. he's more old-fashioned, like legends as sergio leone. but in this section, you won't find many who reach his level of cinematic genious.