It depends who you ask and when.
I remember being blown away by ESB when it first came out and instantly thought it was superior to ANH in every way.
Naturally ESB wouldn't make any sense without ANH but it really raised the bar for me.
Where as my cousin who was two years younger than me though that ESB was frustrating at the time because the Millennium Falcon kept 'breaking down' and the bad guys seemed to win but now he prefers ESB too (though he also likes the PT).
When I took him to see ROTJ in 1983 he thought that was the best one where I thought it was mostly rubbish, he now ranks it between TPM and AOTC.
Fastforward to 1997 and I took my niece to see the SEs and she loved them all pretty much equally.
Around friends of the same age as myself they always thought ESB was the best.
One of them had missed ROTJ and the PT but his nephew, my godson got a lightsaber for his birthday and wanted to see the films.
He couldn't believe how crap they were including ROTJ but the kid loved all of them and really liked The Clone Wars too.