You would need to look for a blue subway line because the 8th ave. The longest subway line is the 8th A train. This train travels from the shore of the Atlantic Ocean in Far Rockaway Queens to the hills of Washington Heights way up on 207th street. The A train is express during the rush hours, making limited stops in Brooklyn and in midtown Manhattan. The A train makes the longest express stop from 59th street to 125th street.
Every subway line has some sort of tourist attraction close by.
The 6th line at 34th street is home to Macy's Department Store the largest department in the world. Plus it is only 2 blocks from the Empire State Building. The Broadway line is close to most Broadway theatres. The 8th avenue will drop you off at Central Park or the World Trade Center. The 7th avenue is going to take you South Ferry where you can board ferries to Ellis Ilsand or Staten Island.