Because you take a lot of things from ESB for granted ... for example the "I am your father" thing is easy to downplay now that it's been copied/made infamous for 35 years at the time it was ground breaking. 

Also ESB deserves credit for YODA ... that was the movie that made Yoda, now you take the character for granted because he's "just Star Wars", at the time that was a huge risk to make a puppet a major dramatic character. Look at Episode I for how big of a disaster things can be when your "gimmick" character (Jar Jar) doesn't work. 

Empire MADE the Star Wars saga. The first movie is just a simple adventure film with little depth. The second film basically is what created the darker more pyschological aspects of the Force and the entire saga for that matter, it's also the least cartoony/silly of any of the Star Wars movies and has the best performances from Harrison Ford. 

It introduces Yoda, the Emperor, Boba Fett, Lando Calrissian to the saga. None of these characters existed before. 

If Empire had sucked/been a flop, there probably really is no Star Wars saga, it would've just been a one-off thing that was fun the first time but never gained any legs after -- see The Matrix trilogy and how the dissapointing second one basically sank the entire franchise. 

Also A New Hope is by far the slowest paced Star Wars movie by far. Empire hums at a good rate as the heroes (at least Han/Leia) are constantly being chased by the Imperials. 

1 . Empire Strikes Back
2 . Revenge of the Sith
3 . The Force Awakens
4 . The Phantom Menace
5 . Return of Jedi
6 . A New Hope
7 . Attack of the Clones

To be fair though I like them all. Except for the nonsense parts like Ewoks, Jar Jar or Anakin flirting.

1) Revenge of the Sith
2) A New Hope
3) Return of the Jedi
4) Force Awakens
5)Empire Strikes Back
6) Phantom Menace
7) Attack of the Clones.

About on par with the original trilogy

1. Revenge of the Sith

2: The return of the jedi

3: The Force Awakens

4: Empire Strikes back

5:Attack Of the clones

6:The Phantom Menace

7: A New Hope

1. A New Hope
2. The Force Awakens
3. Empire Strikes Back
4. Return of the Jedi
5. Revenge of the Sith
6. Attack of the Clones
7. The Phantom Menace

1: the Empire strikes back
2:return of the jedi
3: the force awakens
4:revenge of the sith
5: the phantom menace (the Saber fights puts this above new hope.)
6: A new hope
7: attack of the clones

1) the force awakens
2) the empire strikes back
3) a new hope
4) return of the jeditor
5) revenge of the sith
6) the Phantom menace
7) attack of the clones

Lots of Revenge of the Sith love. Glad to know i'm not the only one.

1. Empire Strikes Back
2. The force Awakens
3. revenge of the sith
4. return of the Jefi
5. A new hope
6. Attack of the clones
7. The Phantom Menace

Updated after I saw The Force Awakens a second time. TFA gained two spots and ANH also.