I’m a big fan of The Who, but if I had to recommend just one of their albums for someone who had missed out on a lot of rock music I’d go with the already-mentioned Who’s Next. There are other albums that are more significant to me as a Who fan, such as Quadrophenia, but at 81 minutes Quad would be a long slog for someone who turns out not to like it. It’s also not going to expose you to any of the band’s biggest hits. Who’s Next is a classic album that contains several of The Who’s most famous songs, so even if (blasphemy!) you wind up hating it then at least you’ll be familiar with some of the Who songs that “everybody” knows.
More suggestions that deserve a must-listen for the OP, since it seems he’s dialed in his taste a bit better in this thread now…
Billy Idol - Rebel Yell – A pop-glam-punk masterpiece, great catchy powerful songs.
Creedence Clearwater Revival - Cosmo’s Factory – Great soulful rockin’
Van Halen 1 --This is the Appetite For Destruction of the 70’s, a first album from out of nowhere that gobsmacked the guitar world.
Nirvana - Nevermind – Well-deserving of its revolutionary establishing of a new rock genre. Great catchy songs throughout.
Rush - 2112 – Bombastic arena art-rock at its best. Geddy Lee’s voice is an acquired taste that many fail to gel with, but definitely give it a run.
Jane’s Addiction - Nothings Shocking – Encapsulates the sound and feel of the 90s for me.
IMHO Rush 2112 is not the Rush album to recommend to a newbie. Moving Pictures is much more accessable.
I agree. The concept of 2112 is probably too much for someone who hasn’t listened to Rush.
I would also recommend Badmotorfinger by Soundgarden (Not sure if it’s been mentioned yet. It probably has.) In retrospect, I think Soundgarden was probably the best of the bands to come out of the Seattle scene in the late 80s-early 90s. I like Nirvana a lot but if given a choice, I would probably pick Badmotorfinger as the album to listen to from that era/genre.
Superunknown and Dirt.
My Wave seems to get to me, as does Down in a Hole and Rooster.
You guys are getting to where I am already. I started with Nirvana, followed Pearl Jam, and Alice in Chains is my favorite band(of that time, anyway).
I’m not a big Soundgarden fan, though.
Have you heard All My Life by Foo Fighters? Not a great album, but damn that song…and did you listen to any Ministry?
Don’t know if this might seem a bit out of left field, but how about The Pretenders: Learning to Crawl? Good, strong, straightforward post-punk/new wave rock with Chrissie Hynde’s distinctive vocals. Their first two albums were strong, too, but this one is their first after the loss of two band members, and it’s notable for being a successful comeback/continuation of the band.
OK, I give up. 
Did you ever listen to Jane’s Addiction?
Oh, I agree Superunknown is great too. My faves:
My Wave
Fell on Black Days
Superunknown
The Day I Tried to Live
4th of July
Like Suicide
In case anyone here missed it, a link to the current Fleetwood Mac/Rumours thread seems appropriate in a thread about albums that everybody has heard.
I’m a teacher in a new job and work has just picked up x10. I’ll keep updating for time to come, but with less frequency.
I’ve listened to one album, though not a classic one, since last time.
NIN’s Downward Spiral(1994) is just when I began shifting musical tastes. I don’t actually hail that album as much as some, but do love the following tracks: Reptile(amazing song), March of the Pigs, and Hurt. Other tracks are fine, but those are what I loved. I went back and listened to Pretty Hate Machine(nice) and Broken(pretty good, too).
So, a bit of a cheat since it is new, but:
**Update:
NIN - Hesitation Marks**
Wow, this is actually a really good album. I did not enjoy much of How to Destroy Angels. That was an anticipated project for me and was a huge letdown.
This, however, was not anticipated by me at all. I had not been paying attention and only learned of the album through Entertainment Weekly’s review.
It’s great. It’s not an album I’d thrust up into the top level of great albums, but has some of the best NIN stuff I’ve heard since Downward Spiral. I won’t go track by track, but I really recommend pretty much the entire thing. Reznor is wise to not churn out albums too quickly. The risk is that we wait years for an album and are disappointed(which I have been with NIN lately…like the last 10 years or so). However, this time, the album is solid and should elevate NIN back onto the radar for some folks.
It is also listenable for the average non-fan, something that makes it better for regular people like me who are not huge fans.
Oh, and it skews very far from the depressing content of most of his albums. Reznor doesn’t sound exactly happy, but it really is a different vibe than previous work. Folks who appreciate the gloominess of NIN may be let down, but I didn’t mind at all.
I don’t always like his music, but I respect Reznor and this time, I am in agreement with his choices.
Buy Hesitation Marks, folks. It’s worth it.
Grinning while reading this thread (I’m a teacher and a big fan of music)
‘Thumbs Up’ for every band highlighted here, but especially The Who (‘Who’s Next’), Led Zep (‘Remasters’), The Rolling Stones (‘Greatest Hits’ is 3 discs), and AC/DC ('Highway to Hell). I want to add so much but don’t want to overwhelm you, so my extras are Pearl Jam’s ‘Ten’ and The Arctic Monkey’s ‘Whatever People Say I Am, That’s What I Am’ which are both very stimulatingly guitary.
Genesis: A Trick of the Tail . One of the best in the post Gabriel era.
Fleetwood Mac-Tusk. Most people will say Rumours, but Tusk is a great, great album
Cream-Disraeli Gears. This why people said Clapton is God.
Jethro Tull-Thick as a Brick. Try to find pictures of the original album artwork online, simulating a newspaper.
A few I don’t think have been mentioned:
KISS: Alive! - Every song on here blows away the studio version.
Montrose: Self-Titled Debut - The first album Sammy Hagar sang on. If you like this, you should check out some of his pre-VH solo stuff, especially Standing Hampton and Three Lock Box.
And not to complicate the AC/DC issue, but I would recommend Powerage, the one before Highway To Hell. Very raw and unpolished, and the better for it.
This is an interesting thread - sticking with the notion that new ears can honestly reassess the commonly held classics, try these:
Steely Dan - “Aja” has that whole fin de cicle, bi-coastal, 70’s dark side thing down better than any record I can think of - you may find it an interesting change of pace from some of the recommendations
Miles Davis - “Kind of Blue” - I’d be interested to see what you make of some classic jazz
Lucinda Williams - “Car Wheels on A Gravel Road” - what do you reckon of what was the future of country music/Americana, 15 years ago…
Good man. 
I especially enjoy the second disc of Quadrophenia. Still, The Who Live At Leeds stands tall as the quintessential live rock album.
Geeky stuff. Quadrophenia was released in four channel stereo for the new quadroponic stereo systems being devised by Sony, Pioneer, and Kenwood in 1972. Pink Floyd joined in with Dark Side of the Moon. I remember as a youth oogling quadrophonic systems with their concrete base turntables knowing I could never afford such high technology.
Today…you can buy DVD players with 5.5 surround sound from a chain store. I’m not sure whether to weep or smile. The word Quadrophonic still sends goose bumps up my spine.
**Update:
Fleetwood Mac - Rumors**
Well, I had of course encountered “Don’t stop thinking about tomorrow” and not realized it was by Fleetwood Mac. I think Clinton even used it as his campaign theme or something. Probably a fine song, but it gets stuck in my head and I don’t care much for it.
Anyway, on to the album.
OK… I liked it quite a bit, though I would not give it a raving review.
I think Go Your Own Way is an amazing song. Really, it’s a great piece of song writing and is well performed on the album. It must make a great live performance. Certainly the high-point of the album and it must get covered quite a bit.
Dreams is also an excellent song and I find myself listening to it frequently.
I like a few of the other songs, but none as much as those two. You Make Loving Fun is also quite good and I think the album is very strong.
I give the album high marks and I’m keeping it on my Ipod, though I do expect to skip a few tracks every time I listen to it. In fact, I may gradually delete tracks to make a shorter EP length album out of it.
It’s an album that I think gets better when you know the backstory of the band, especially what was going on with Stevie and Lindsay and their breakup. It’s why songs like Go Your Own Way resonate so strongly, they are coming from a very real place.