Why do you think the love scene in Capulet's garden in Shakespeare's Romeo and Juliet is the most famous one in all of literature?

It may be a bit of an exaggeration to say that the Capulet garden scene in Romeo and Juliet is the most famous in literature, but it is definitely well-known.  I think the reason for this is that the story itself is compelling, because of the forbidden love.  Also, the language is beautiful and perfectly captures young love.

One the most interesting things about this scene is that it really only says that Juliet is at a window.  She is usually given a balcony so that she can actually move around.  There is limited movement at a window.  Also, Romeo could see her better on a balcony.  I think there is something inherently romantic about balconies, anyway.

The story of Romeo and Juliet is compelling, of course.  This would not be one of Shakespeare’s most popular plays if it wasn’t.  In this scene, we get to see the young lovers agree that they will marry, so it is the culmination of their brief courtship.  Romeo also demonstrates how much he loves Juliet by comparing her to the moon, which he says she outshines.

ROMEO

He jests at scars that never felt a wound.

JULIET appears above at a window

But, soft! what light through yonder window breaks?
It is the east, and Juliet is the sun.
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief,
That thou her maid art far more fair than she:
Be not her maid, since she is envious … (Act 2, Scene 2) 

Juliet has her share of interesting and memorable lines too.  For one thing, she seems to ponder Romeo’s very existence, including whether she can really love a Montague.  Like Romeo’s lines about her, she says these things in our hearing.  While Juliet cannot hear Romeo, Romeo may hear her.

Tis but thy name that is my enemy;
Thou art thyself, though not a Montague.
What's Montague? it is nor hand, nor foot,
Nor arm, nor face, nor any other part
Belonging to a man. O, be some other name!
What's in a name? (Act 2, Scene2)

From a theatrical perspective, this conversation is interesting because both are speaking but not speaking to each other.  The audience also gets to hear Romeo and Juliet’s innermost thoughts, like seeing into their hearts.  For those reasons, the scene is memorable.