How's this for diversity... these are all albums I HAD to have on my iPod when my wife got me my 80 GB last Christmas:
Moody Blues: Days of Future Passed (<-- the full length version of Nights in White Satin is the most majestic song ever)
Miles Davis: Kind of Blue
Alison Krauss & Union Station: Live
Entire Pink Floyd and Led Zeppelin catalouge (honestly, how can anyone go wrong there?)
Rolling Stones: Sticky Fingers and Exile on Main Street (I'll never get tired of Can't You Hear Me Knockin, Wild Horses, Tumbling Dice, and Sweet Virginia)... in fact, I got Fourty Licks because they just have too many good songs not to get the most up to date greatest hits collection
The Beatles: Help (you gotta Hide Your Love Away) and the White Album (while my guitar gently weeps)
Any collection of Tchaikovsky works (Nutcracker selections, the 1812 Festival Overture, it tickles my inner culture-snob, what can I say)
There's tons more. I could go on and on about what I've got on my iPod
