For me, I’ve gotta say “Machine Messiah”, by Yes. It’s one of the longest, and most interesting songs I’ve heard. Most songs have just one “mood” explored in them, but this one shifts from joyous to melancholy and back multiple times, and does so perfectly. Plus, I’m very nostalgic about it since it was one of my favorite songs as a child. Drama was the one of the first two albums I bought as a kid, almost entirely because that one song was on it. (Also–is that a Sonic pfp, op? A person of culture, I see 😉)