For me, I think it’s Alice in Chains Again.
Not necessarily my favorite tune (but pretty close), but it has a certain quintessential gritty, drudging, driving, and raw energy to it.
That’s a really good one. You know, as the years have passed, when I think of “grunge”, I really don’t think of bands like Pearl Jam and Nirvana in that group anymore. I tend to gravitate more towards bands like AIC, early Stone Temple Pilots and Soundgarden. Since you already nominated an AIC song, I’ll say Soundgarden’s “Burden In My Hand”: https://www.youtube.com/watch?v=gAfPZp5Rhjo
I do have an extreme soft spot for AIC’s “Would?” though. Probably my favorite song from that era, and one of my favorite songs of all time. Very haunting, particularly in light of Lane Staley’s slide into heroin addiction and death.
I’ve been listening to Soundgarden’s Superunknown with my guitar-playing son lately. Jeez, what a great album - although if they were going to name it after a song, it shoulda been called Black Hole Sun ;).
I love how un-heavy some of Kim Thayil’s riffs are, and yet how heavy the overall song sounds. The chorus of My Wave, heck, little stabby guitar bit when Cornell says “don’t come over hear, piss on my gate”. Most songs don’t have big blocky “hard rock” riffs except for Spoonman - where they added…spoons. I just love how they create a big, heavy space they build together in their riffs, if that makes sense.
But I don’t find them jumping to mind when I think grunge - when I want a grunge jam, I go with Pearl Jam’s Alive. I am much more of an AIC guy (btw, their grungiest songs, to me, are Down in a Hole and Would?) and a Soundgarden guy - but Alive is my quintessential grunge song. Nirvana is in its own space - like it or not, they are the Beatles of grunge, just like Metallica is the Led Zeppelin of Thrash 
Mudhoney’s “This Gift.” Heavy, slow, nasty and, well, grungy.
That’s a very good way to place Nirvana in my mind. I agree with “Alive” to the extent that it was one of those “Where were you when THAT song first started playing?” but Pearl Jam to me was…how to put this…just not as “dark”? They did have some dark lyrical content but they never really gave me that “depressing” vibe that came off bands like AIC in waves.
I suppose that’s not really a requirement for grunge, but hey…sticking with it!
Kim Thayil is kind of like the early 1990’s grunge version of Tony Iommi.
That’s the one I thought of when I read the thread title. Great riff, it just pounds along.
Soundgarden-“Outshined”
Let me offer up STP Sex Type Thing and Temple of the Dog Pushin Forward Back.
I probably would have picked Again as well.
ETA: I forgot November Hotel by Mad Season. It’s a slow burn but I love it.
“F*!#in’ Up” by the Grandfather of Grunge.
I’d go with Birth Ritual by Soundgarden, Interstate Love Song by STP or Come as You Are by Nirvana.
I agree with some flux in some own idiosyncratic way in how my brain arranges these bands in my head.
And speaking of Soundgarden, “BlackHoleSun” is usually my gamer handle. Amazing album. 
For me, Nirvana spun the whole thing off over the face of what rock was and where it would go over the indefinite future. Kurt was coming from a stripped down place. It felt like Kurt’s band, with Dave and Krist in tow (though what a trio!). In my head, I have to keep them in the grunge genre, but my love for their music transcends grunge in ways Soundgarden, STP, AIC, etc never will.
As for Pearl Jam, I stuck with them all the way to Binaural, but agree that Eddie’s music, while stuck in the grunge genre for lack of a better word at the time, just didn’t have the bile his contemporaries did. His more emotional, mellow stuff always worked the best for me.
"Would,“Them Bones,” “Dam that River,” and “Brush Away” are more top of my list AIC favs.
“Radio Friendly Unit Shifter” by Nirvana always makes me wanna kick over a few tables too.
Pretty much ALL of “Dirt” is…grungy?
Soundgarden’s “Jesus Christ Pose” for me. Just epic. Epic. With one of the best grooves in grunge drumming.
As for the Nirvana side discussion, despite the fact that they broke the whole grunge scene into the mainstream with “Smells Like Teen Spirit,” they were always more of a punky college rock band a la the Pixies to me than the dirty, grimy, swampy, Black Sabbath-influenced rock that I think of as grunge. It’s all a continuum, and they were definitely on the pop-punk side of the grunge continuum, as opposed to the more 70s metal, almost “proggy” (as in prog rock) side of grunge that Soundgarden and Alice in Chains ventured into. Pearl Jam would be somewhere smack dab in the middle, I think.
SoundGardens Gun. Best Mosh Tune EVER!
“100%” by Sonic Youth came to mind immediately. I went and watched the video and it was mystifying. The band looks out of place at a house party with a flannel dress code-- one young woman has a flannel bikini top-- while shots of an estatic Jason Lee on a skateboard pop up. I still like the song, just now I feel weird about how music I thought was at least a little rebellious was marketed.
Hey, L7’s “Andres” really shrieks and grinds. Let’s not watch the video this time.
STP is just fucking awesome.
The other day I had ten minutes to kill before my next TV show came on. I headed over to YouTube because I was jonesing for that song: “Where the river goes”.
What was supposed to be a ten minute time killer turned into an hour or so ordeal as I wound up listening to that entire album. (The entire album is up on YT)
Also, Alice in Chains: Rooster.
Alice in Chains is a pretty big cut above the rest in the grunge scene, to me.
I’d have to say “Them Bones” is the perfect Grunge jam.
And since I love AIC so much, I’d also suggest just about any live performance of “Bleed the Freak”. WHAT a song!
Dirt by AIC.
Purple by STP.
Nevermind by Nirvana
Ten by Pearl Jam
and Superunknown by Soundgarden.
I mean, really, hardly a song to skip on any of those 5 seminal grunge albums.
I’d have to go with Smells like Teen Spirit, which I’m sure is too mainstream, but it’s just The Song from that era to me. I can’t turn off the radio in the middle.
AIC’s Would is also a good choice. Or Pearl Jam’s Black. Or Black Hole Sun.
And now I’m sad that I have no good music on my phone.