Country - Restless by Carl Perkins
I honestly have no idea what you should have here, but I will suggest Carl Perkins (probably more rock than country).
Classical - try Shostakovich or Wagner
I like these composers but I haven’t explored classical much. I know I like certain pieces, but then I’d have to track down the composer, conductor and orchestra in order to find the music I heard. A daunting task indeed.
Funk - Funktelechy vs. The Placebo Syndrome by Parliament
These guys really are from another planet.
Hip Hop/Rap - Midnight Marauders by A Tribe Called Quest
You probably have this already.
Pop - Off the Wall by Michael Jackson
Remember a time when MJ had a 'fro, Quincy Jones was his producer and he turned out HOT albums like this.
Dance - Mixed Live, Crobar Nightclub Chicago by Carl Cox
For pure high energy dance beats look no further than three turntable master Carl Cox. I’m sure he has better albums out there, but you can’t go wrong with this one.
Disco - The Best of Cheryl Lynn - Got to Be Real by Cheryl Lynn
My favorite disco diva.
World - Calypso Carnival by The Mighty Sparrow
I suppose I should recommend Bob Marley or something, maybe some Brazilian or Japanese drummers, but instead I will shamelessy plug an artist from the home of my ancestors, THE MIGHTY SPARROW! Calypso Carnival is a good but quite old album. You may also try looking for artists such as Crazy, Tall Paul and Square One. Most of my stuff is on mix tapes or CDs so I don’t know how much luck you’d have finding these artists on iTunes or in a record store.
R&B Street Songs by Rick James
RICK JAMES!
Jazz - Birth of the Cool by Miles Davis
I think Kind of Blue is more often recommended but I prefer Birth of the Cool as The One Jazz Album You Must Own. Better yet get both. Alternatively you can go with John Coltrane’s soulful A Love Supreme.
Gospel - O Brother Where Art Thou Soundtrack
I’m not a fan of most gospel, but this soundtrack has a lot of good old-timey spirituals on it.
Easy Listening - What the fuck do you want this on your iPod for?!
Soundtracks - The Collection: 40 Years of Film Music by John Barry
Soundtracks are kind of hit and miss, but this collection has some of John Barry’s best work covering a broad range of styles all performed by the same orchestra. He is one hell of a composer.
You should also try some J-Pop like Pizzicato 5 (Made in the USA, Playboy Playgirl) and lounge artists like Thievery Corporation (The Cosmic Game, The Richest Man in Babylon), Swayzak (Himawari) and Jazzanova (In Between, The Remixes 1997-2000).