I basically think that doubling lead vocals an octave above or below, and appreciably quieter than the lead, makes any recording monotonically better, and is neither here nor there in terms of “interesting”:

• Oingo Boingo – Dead Man’s Party (chorus) (octave below)
• Guns N’ Roses – Knockin’ on Heaven’s door (octave above)

I would conjecture that for all songs in the universe, you could add a double'd vocal and either turn it up so as to be perceived, or turn in down so as to be imperceptible, and one of these would make the song better. It's like make-up. Or almonds. Or profanity.

Remind me someday when I run a giant studio to always have every vocalist record an octave above and below the lead, just in case.

Speaking of Dead Man's Party, Dead Man's Party has unambiguously my favorite horn-section solo ever, which is a low bar, since I couldn't think of another horn-section solo I care about:

Oingo Boingo – Dead Man's Party (horn solo)

Depending on your visceral reaction to trashy country music, you may think that this is an exception to the "doubled > otherwise" rule:

Big & Rich – Save a Horse, Ride a Cowboy

The redneck at my core doesn't mind it though.

True story: I was driving to a soccer game in Seattle one night... probably an hour in the car one way, 30 minutes the other. 90 minutes total in the car. A local country station was doing some very bizarre promotion where they played "Save a Horse Ride a Cowboy" non-stop for 24 hours, no commercials, no other songs. I listened for the full 90 minutes I had to devote to this project.

So yes, I'm still all-in on octave-doubled vox.

Today I came across a rare instance of octave-*tripled* vocals:

Guns N' Roses – Locomotive (pre-chorus)

Also, IMO Use Your Illusion is the best rhythm guitar album in the history of rock. I can't get enough of the guitars, Izzy on one side and Slash on the other, both constantly doing interesting things and so cranked that they're *just* on the safe side of feedback.

Then I also came across a less-common case of octave-doubling, with the new octave is *below* the lead (like Dead Man's Party), which in this case sounds ridiculous on purpose:

Mötley Crüe – Smokin' in the Boys Room (chorus)

You'll get the most out of hearing the lower octave if you're watching the video. Or, if you prefer, 2 minutes later:

Mötley Crüe – Smokin' in the Boys Room (last chorus)

Simpler times, the '80s were. Simpler times.