My bid is for the most overrated classic rock song is Kashmir by Led Zeppelin. I thought it was droning and monotonous in the 70s and I still do. It's mainly a simple ascending riff that goes "duh duh duh- duh duh duh- duh duh duh- (repeat riff only on higher notes) duh duh duh- duh duh duh.... for 9 minutes And before anyone mentions is, yes Beethovens 5th Symphony opens with a very simple riff too, but he at least managed to keep it brief and inject the rest of the symphony with lots more variety. Kashmir has always sounded to me aloof, robotic and very, very boring. By 1975 Plants voice was pretty much shot too, so the vocal delivery is full of whiny and cracked notes. The middle section is a welcome relief and a good performance from Bonham, but he can't save the song.
And it's not so much that, it's the fact that it seems that two thirds of every hard rock band since have felt the need to do their own version of the song at some point on one of their albums. There's so many songs I've heard that have been obvious attempts to emulate the "duh duh duh- duh duh duh" riff. I can't help but think it's something that bands think they're "supposed" to do if they want success.
Runner up- Another Brick In The Wall. This is so untypically Pink Floyd. It almost sounds like a funky disco track! Maybe that's why it was a success. Or it could be the "we don't need no education" chant became a favorite of rebellious schoolkids. I certainly remember it being popular at my school in 1979!