I would say that the classic rock instrumental would have to be either La Villa Strangiato or YYZ.
But that's a Rush fan for you.
I am the biggest Rush fan here, but neither of those are regularly played on classic rock radio everywhere, thus taking them out of the equation.
Well, if they have to be played on the radio....
Summer Song.
While classic rock radio has expanded to include the early 90s, judging by the near constant playing of Soundgarden, Rage Against The Machine, Nirvana, and Alice in Chains on my local classic rock station-Summer Song isn't in the same league of iconic as Frankenstein.
As for Sweet Emotion, it's hard for me to talk about Aerosmith without talking about the girl I had a crush on in high school, and who I wound up meeting a couple of years ago and actually talking about Aerosmith's 80s music for a time, but I'm going to give it a shot.
Sweet Emotion is largely the overlooked song from Toys in the Attic, with Walk This Way getting the most attention, but Sweet Emotion has a great riff driving it, and has a lyric that I imagine a lot of young people have to figure out by guesswork, since "the rabbit done died" has been replaced by "the strip turned blue" to indicate a pregnancy test. Actually, thinking about it, given the usual lyrics on Aerosmith songs in the 70s, I have no idea why that lady I know loved them so. These days they'd be hammered into the dirt for being sexist. Times they have changed.