Everyone needs some Zep, Cream, Jimi, Doors (Robbie Krieger is way underrated!), Allmans, Santana, Sabbath ... I'd add some Blue Öyster Cult to that list, too ('cause Buck Dharma is also underrated!), probably some Mountain, Skynyrd .... AC/DC, Maiden, Motörhead, The Ramones, Neil Young (I like the electric stuff better, like Live Rust or Weld ....
But those are just, like, the classics of rock, after all!
I'd almost be tempted to say that one needs the Zeppelin DVD set as much as anything else. A very visual band, and there are some great performances on there. It will make you feel very, very good about having a Mule-loaded LP, too :)
Naturally, one needs some Gov't Mule. I like the older stuff with Alan Woody on bass better, and I might plunk for the Live at Roseland disc, which was a mid-90s New Years show when they were still playing support, if I recall aright. It's worth it for the version of "Mule" alone!
I'd also recommend some Bruce Dickinson solo stuff as much as any Maiden, frankly. I think The Chemical Wedding was one of the best albums of the '90s (or any decade, really) and the guitars are amazing. (Speaking of old dudes doing good, I think the new Iommi/Hughes Fused album is well worthwhile, too.)
And speaking of great '90s albums, I'd put Amorphis's Elegy on that list. I'm not much of a death metal fan, but I thought Elegy was a perfect balance between the band's death-metal roots and their lighter, proggish aspirations. Which reminds me that The Gathering's Mandylion is well worth having, too.
I must put in a word for Jethro Tull! Tull's catalogue has a lot of stylistic variety, and some qualitative ups and downs, but from a guitarish perspective I feel good about saying Benefit rocks :). It's from the same year as Sabbath's first album, and actually gives me a similar vibe in many ways. (Martin Barre is also underrated!)
And though they're terminally unpopular/unknown in the UK, I'm a good ol' Grateful Dead fan, and think Europe '72 offers a reasonable balance of showcasing their songwriting and their Coletrane-on-acid style improvisational jamming. Though almost as with Zeppelin, I think nothing shows off that band as well as The Grateful Dead Movie (recently released on DVD).
Richard Thompson is great, though I'm hard pressed to think of an ideal album to show him off.
Ah, well, with all the excellent recommendations people on this thread have had, you can't go wrong!