What are the points of interest at each stop on the MAIN line of the NYC subway? (include reasonable walking distance)
Which line, the 1 or the 4?
If 1=the biggest, then 1.
There is no single main line on the NYC subway. Look at the map and perhaps choose one that seems long or interesting. If you’re going to be stopping on and off often, you could also choose a route that combines portions of several lines.
As waterj2 notes, there is no main line of the NYC subway. Your best bet is to pick a line and find stuff that looks interesting that’s close to it. There’s a lot of museums on fifth ave between 80th and 105th streets (museum mile (actually a bit more than a mile) 
). They are accessable via the Lexington Ave (4, 5, and 6) trains which are two blocks east of Fifth Ave.
There’s a lot of cool tourist stuff in the Times Square/Broadway area (including the theater district.) That’s available via the 1, 2 and 3 (7th ave) and A, C and E trains (8th ave) and the N, R and W trains (Broadway.)
The Museum of Modern Art is currently in Queens, accessable via the 7 (Flushing) train.
I would recommend “The Underground Guide to NYC Subways” by Dave Frattini. It has info on every station, including history, architecture, nearby points of interest, even eateries. There are some omissions and inaccuracies, but in general it’s a pretty decent guide.
As others have said, there is no main line. But there’s a lot of good touristy stuff along the greenies (4, 5, and 6) which takes you along the east side of Manhattan. Off the top of my head…
friedo mentioned Museum Mile, the most recognized museum on that stretch is the Met off of 86th st. Walking a bit in the other direction is Gracie Masion.
From 59th St. you can check out Bloomie’s (the department store, not the mayor) and can see the famous Citicorp building. Its also within walking distance of FAO Schwarz and the southern entrance to Central Park.
St. Patrick’s Cathedral is close to the 51st street stop, as is Rockefeller Center.
The 42nd St. Stop is Grand Central and you can walk northeast to the UN.
14th Street is Union Square which has a park, the Virgin Megastore and a lot of flea/farmers market type deals.
8th Street/Astor Place is good if you wanna check out subversive pierced types along St. Marks. IIRC, it’s also near the precinct house shown on NYPD Blue.
Canal St. has Chinatown.
Brooklyn Bridge - City Hall has, well, Brooklyn Bridge and City Hall.
If you take the 4 waaaay up north you can see world’s greatest baseball stadium and the world’s best zoo. But neither are in the world’s safest neighborhood, so I’d recommend a native guide accompany you.
Of course there are other interesting spots on other lines, and there’s really no reason you need to commit to one line.
You must take the A train
To go to Sugar Hill way up in Harlem
If you miss the A train
You’ll find you missed the quickest way to Harlem
Hurry, get on, now it’s coming
Listen to those rails a-humming
All aboard, get on the A train
Soon you will be on Sugar Hill in Harlem
I was going to do the 2 and the 4 trains, but I’ve got to go and this took longer than I thought. Note: The D train service is distrupted. It doesn’t run like this now, but it’s supposed to.
D–
Fordham Road-- the largest shopping district outside of Manhattan. The zoo and Fordham University on one end and Jimmy’s Bronx Cafe at the other.
161st and Yankee Stadium.
Yankee Stadium, of course. And if you get in a drunken brawl there, the county courthouses are right up the block.
59th Street and Columbus Circle.
The Ritz, horse drawn carraiges at Central Park, the Merchant’s Gate, Grand Army Plaza.
47th Street- Rockerfeller Center.
Radio City Music Hall, The Skating Rink, The Diamond District.
42nd and 5th.
New York Central Library
34th Street.
Herald Square, Empire State Building, Macy’s
West 4th
Washington Square Park, NYU
Grand Street.
Chinatown
Dekalb
Junior’s
Atlantic Ave
BAM
7th Ave.
Grand Army Plaza, Prospect Park, Central Library.
Sheepshead Bay
Manhattan Beach, Lundy’s, Pips
Coney Island
NY Aquarium, Keyspan Park, Coney Island.
That mysterious space between West 4th and Grand Street in Biggirl’s post is where the Broadway-Lafayette stop goes.
Gateway to the East Village, SoHo, Little Italy, the Puck Building, and Dean and Deluca’s.
Forgot Brighton Beach in between Sheepshead Bay and Coney Island too.
The stop is called Brighton Beach. There’s a beach there.
Hmmm. S’mothers:
Manhattan:
A @ 181st: Cloisters (branch of Metropolitan Museum - incorporates elements of five medieval cloisters brought over from Europe.)
1/9 @ 116th: Columbia University
B/C @ 81st: American Museum of Natural History
1/9 @ 79th: Zabar’s (yet another famous foodie heaven - but reasonably priced)
7 @ 5th Ave: New York Public Library (with lions “Patience” and “Fortitude.”)
4/5/6/7 @ Grand Central: also close to Chrysler Building
A/C/E @ 34th: General Post Office, with the inscription
“Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds.” Facing post office: Madison Square Garden. Underneath MSG: the remains of Pennsylvania Station.
N/R @ 23rd: Flatiron Building
1/9 @ Christopher Street: Stonewall Inn bar, site of 1969 riots.
E@World Trade Center: Ground Zero. Station still bears the WTC name. Also St. Paul’s Chapel (“GW Prayed Here”).
A/C @ Chambers St./1/9 @ Park Place (two connected stations): Woolworth Building.
J/M/Z @ Broad Street: New York Stock Exchange, Federal Hall (“GW Inaugurated Here”).
N/R @ Whitehall/4/5@Bowling Green: Old Customs House-Smithsonian’s National Museum of the American Indian (fantastic collection); Fraunces Tavern (“GW Said Goodbye Here”)
1/9 @ South Ferry: Battery Park, The Sphere (idealistic statue of globe, partly destroyed on 9/11 and rededicated as memorial); Staten Island Ferry terminal, Liberty/Ellis Island ferry docks.
Brooklyn:
A/C @ High Street: footpath for Brooklyn Bridge - the walk over the bridge to Manhattan is, IMHO, NYC’s most spectacular walk.
3/4 @ Grand Army Plaza: Brooklyn Botanic Garden. One of the best around.
3/4 @ Eastern Parkway: Brooklyn Museum. One of the world’s greatest collections, but often overlooked because of Met.
I played saxophone for 16 years. I’ve played both lead and second, both alto and tenor parts in big band, jazz band, combo, fake and real book arrangements of that tune dozens of times, and at one point I could have recited the changes for you.
Can you believe I never knew it actually had words? hahaha
Your best bet, though, is to pick an area and just walk around. Why bother getting on and off the subway every other stop? Unless you have physical difficulties, you will find more than enough points of interest within walking distance of each other.