It's not even in my top 5 Nirvana songs. The riff you're talking about is Boston's "More Than A Feeling" --- so much so that they used to play a fake-out of "More Than A Feeeling" in concert. I like "Smells Like Teen Spirit" more than "More Than A Feeling", but not like, much more.

1. Where Did You Sleep Last Night Unplugged

2. Frances Farmer Will Have Her Revenge On Seattle (or, interchangeably, "Rape Me", though the "blanket of ash" line gets me every time with Frances Farmer).

3. Lithium

4. Heart Shaped Box

5. Dumb (unplugged).

It's not fair to make the #1 song a cover, but that performance merits it.

It was culturally influential, signaled the end of glam rock, and was even dubbed the anthem of a generation, but it didn't showcase great songwriting. It's not fair because he had time to grow as an artist, but "Everlong" is a better song.

It's really twofold. You could also identify the song just from Dave's riffs.