Gotta have some Bone, Thugs& Harmony in there too.
A Tribe Called Quest - Midnight Marauders; The Low End Theory
Nas - Illmatic; Stillmatic
De La Soul - 3 Feet High and Rising; De La Soul Is Dead
Dr. Dre - The Chronic
Wu-Tang Clan - Enter the 36 Chambers; Forever
Raekwon - Only Built for Cuban Linx
GZA - Liquid Swords (NOT Legend of the Liquid Sword)
Black Star - Talib Kweli and Mos Def are BlackStar
Talib Kweli - Reflection Eternal; Quality
Notorious BIG - Ready To Die; Life After Death
Tupac - All Eyez on Me; Me Against the World
Common - Like Water For Chocolate; Be
Arrested Development - 3 Years, 5 Months & 2 Days in the Life Of…
Gang Starr - Moment of Truth
Ghostface Killah - Ironman, Supreme Clientele
Madvillain - Madvillainy
Jay-Z - Reasonable Doubt
Public Enemy - Fear of a Black Planet; It Takes A Nation Of Millions
Boogie Down Productions (BDP) - Criminal Minded; By All Means Necessary
Beastie Boys - Pauls Boutique, Check Your Head, Licensed to Ill
DMX - It’s Dark and Hell is Hot
Blackalicious - Blazing Arrow
Big L - Lifestyles of the Poor and Dangerous
Eminem - Marshall Mathers LP; Slim Shady LP
PM Dawn - Of the Heart, of the Soul and of the Cross The Utopian Experience
Organized Konfusion - Self-Titled; Stress
Souls of Mischief - '93 to Infinity
Masta Ace - Long Hot Summer
Pete Rock and CL Smooth - Mecca and the Soul Brother
Kanye West - The College Dropout
LL Cool J - Mama Said Knock You Out
NWA - Straight Outta Compton
Ice Cube - Amerikkka’s Most Wanted
I dunno how much they count, but I sure love The Streets. The most melodic hip-hop like music I’ve heard. The most melodic actual hip-hop I’ve heard is Kanye West.
I’ve just gotten into hip-hop. I only have a few cds in the genre, but I might get a few Public Enemy ones. There’s one song of there’s I especially like, but I can’t remember the name. It starts out with:
“Bass! How low can you go?”
Rapper’s Delight is also an excellent song.
Those are the strange guys from England right? His flow reminds me of the guy from the clash when he’s “rapping” at the end of one of their songs. (forgot the name, think it’s on Combat Rock)
Anyhoo, you losers have named everything I was going to say, so I’ll have to get obscure on you mofos. 
Down by Law - Murder rap (one of the best songs ever made)
Those are the strange guys from England right? His flow reminds me of the guy from the clash when he’s “rapping” at the end of one of their songs. (forgot the name, think it’s on Combat Rock)
Anyhoo, you losers have named everything I was going to say, so I’ll have to get obscure on you mofos.
Down by Law - Murder rap (one of the best songs ever made)
That’s Night of the Living Baseheads. At its release it supposedly held the record for the song with most samples in it. The video for this song is one of my favourite hiphop videos.
And in case noone has mentioned them (or I missed them):
Killah Priest - Heavy Mental
RA The Rugged Man - Die, Rugged Man, Die
I looked it up, and it’s actually Bring the Noise. I’m not surprised that it’s easy to mix them up though.
World Eater, yep, that’s them. They really grew on me.
Man, I thought I was the only person who liked and/or remembered that album. I know their first album (Reachin’) sold a lot better and earned them a Grammy (ok, technically it was for the single Cool Like Dat, but close enough), but Blowout Comb is just so much better IMO, and would definitely be one of my top 10 rap albums ever.
Qwertyasdfg, I like your list a lot. Some really good lesser known stuff there. But if I can quibble a little bit, while I love the mention of Masta Ace, if I was gonna include just one of his albums it would have to SlaughtaHouse. Also, I like Strictly 4 My N.I.G.G.A.Z. over any of 2pac’s later stuff, but I’m probably deeply in the minority there.
And off the top of my head, some stuff I’d include that hasn’t been mentioned:
Tha Alkaholiks - 21 and Over
King Tee - Tha Triflin Album
The Beatnuts - Street Level
Black Moon - Enta da Stage
Jeru the Damaja - The Sun Rises in the East
The Pharcyde - Bizarre Ride II the Pharcyde
Jedi Mind Tricks - Violent by Design
Atmosphere - You Can’t Imagine How Much Fun We’re Having
Jurassic 5 - Quality Control
The Roots - Illadelph Halflife (significantly better than Phrenology IMO)
Slick Rick - The Great Adventures of Slick Rick
X-Clan - Xodus
Kool G Rap and DJ Polo - Road to the Riches
The D.O.C. - No One Can Do It Better
Compton’s Most Wanted - Music To Driveby
I’d probably think of some more that haven’t been mentioned, given some time, but the above is a good start. Oh, and just to show that I’m not completely out of touch with the current rap scene, here’s a couple 2005 releases that I think are pretty good:
Paul Wall - The Peoples Champ
Three 6 Mafia - The Most Known Unknown
I’ll just add one more ATL artist:
Goodie M.O.B. - Soul Food
Oh, and of course, I post and immediately think of something else I wanted to mention, so…
Gravediggaz - 6 Feet Deep (it was gimmicky as hell, like all of “horrorcore rap”, but it’s still very good)
And on preview, damn, I should have thought of that one, Ogre. I’ve had the Soul Food CD since it came out, and it’s tight.
Actually, I’ll add a couple more (including another -very obscure - ATL artist):
Y’all So Stupid - Van Full of Pakistans
Prince Paul - Prince Among Thieves
Aesop Rock - Labor Days
Would Old Skool fall into the Hip Hop category? If so, I say Gap Band, Roger and Zap, Bar-kays, Slave, Parliament Funkadelic, Lakeside, Confunkshun , Kurtis Blow.
man, we got some heads up in here. good stuff.
I didn’t see KRS-One mentioned (I probably skimmed over it); I Got Next is one of my favorite albums by him.
I have to strongly second recommendations for The Pharcyde and Digable Planets. two of the most chronically and tragically overlooked hip-hop ensembles out there. no one I’ve ever introduced to them has disagreed.
gotta bring it back to Texas though. anybody here ever heard of UGK? those boys can flow. great production on the earlier albums. I picked up Ridin’ Dirty sometime in the 90’s and I never put it down.
last but not least, and my personal favorite, Devin the Dude. you will NEVER hear this guy on the radio. most explicit lyrics you’ve ever heard, not angry though, just right off the tongue like smoke off a blunt. you may recognize him from songs with Scarface, Dre, Too Short, and he was the sole redeeming value on the stupid Jay-Z/R. Kelly crossover album. his self-titled first album is critical to any serious rap enthusiast; even friends of mine who hate rap but smoke a lot of weed can recite every lyric.
Rap isn’t music. It’s a method of delivering lyrics, most commonly associated with hip-hop music. 
Anyway, I’d like to fifth, sixth, and seventh Paul’s Boutique. It’s hugely popular with critics and, judging from the copies of Dust Brother’s-style production seen in the mid 90s, influential with musicians as well. It also has the best music of any rap album I’ve ever heard, as well as the best delivery of lines. Lyrics aren’t so great, but the groundbreaking music is enough to put it in the essential list.
Can’t remember the albums, but:
Young MC
DJ Jazzy Jeff and the Fresh Prince.
These, along with the breakout artists of the early 80’s, contributed immensely to the mainstreaming of rap. There are plenty of people who otherwise look down on rap that like something by these artists (as well as the 5 or 6 breakout raps that started it all in the imagination of America.)
Someone mentioned Sir Mix-A-Lot, most likely referring to Baby Got Back. While I don’t have the album that that’s on, I do have and love SWASS. It stands out as an evolution over most of the rap out there previously, which seemed to either be faithful recreations of a street party (NTTAWWT,) rockers getting in on it for the novelty, dancing posers, or MC’s rapping over studio musicians. SWASS seems to be more along the lines of the Beatles, as an example of the rap artist as a singer/songwriter rather than an MC/DJ.
Hip Hop 101:
2Pac - All Eyez on Me
2pac - Strictly 4 my niggaz
50 CENT- get rich or die tryin
50 CENT-the_massacre
Arrested Development - 3 years
Beastie Boys - Ill communication
Beastie Boys - License to Ill
BEASTIE boys - pauls boutique
BEASTIE boys - to the 5 boroughs
big pun - captial
bone thugs-n-harmony - e1999 eternal from blasphemy
Boogie Down Productions - criminal minded
busta rhymes- the coming
capone n norega - the war report
CEE-lo-cee-lo_green_is_the_soul_machine
Clipse - lord willin
common - like water for chocolate
Coup - Party Music
Cypress Hill - black sunday
D12 - Devil’s Night
De La Soul - 3 feet high
Del the Funky Homosapien - i wish my brother george was here
Deltron 3030
dmx - it’s dark and hell is hot
Dr. Dooom - first come
Dr. Dre - 2001
Dr. Dre - The Chronic
Dungeon Family - even in darkness
Easy E - impact of a legend
Eminem - The Eminem Show
Eminem - The Marshal Mathers LP
Eminem - The Slim Shady LP
EMINEM- encore ~
Eric B & Rakim - Paid in Full
flipmode squad - the imperial
fugees - the score
GAME-the_documentary
gang starr - daily operation
ghostface killah - ironman
Goodie Mob - Soul Food
Grandmaster Flash - message
G-UNIT-beg_for_mercy
GZA - liquid swords
Handsome Boy Modeling School - so hows your girl
ice-t - original gangster
JAY Z- black album
Jay-Z - Hard Knock Life
JAY-z - reasonable_doubt
Jay-Z - Unplugged
Jay-Z- The Blueprint 2
Jedi Mind Tricks - Violent by Design
Jurassic 5 - Quality Control
KANYE west - college dropout
KANYE west- late registration
Kool Keith - Black Elvis
K-OS - joyful rebellion
krs one - krs one
ll cool j - i can’t live without my radio
Makaveli - The 7 Day Theory
Method Man & Redman - blackout
method man-tical
MISSY elliot - the cookbook
Missy Elliot - under construction
Mobb Deep - infamous
mos def & talib kweli - black star
MOS def- new danger
mos_def-black_on_both_sides
Nas - Illmatic
Nas - Stillmatic
NEPTUNES - clones
nerd - in search of
Notorious BIG - Life After Death
Notorious BIG - Ready to Die
NWA - straight outa compton
Outkast - Aquemini
Outkast - ATlients
OUTKAST - speakerboxxx-the love below
Outkast - Stankonia
Peanut Butter Wold - My Vinyl
People Under the Stairs - OST
Pharcyde - Bizzare Ride II
Prince Paul - Prince Among Theives
Public Enemy - It takes a nation of millions to hold us back
puff daddy & the family - no way out
Raekwon - only built
Redman - Whut Thee Album
roots - illadelph halflife
roots - phrenology
roots - things fall apart
Run DMC - Raising Hell
RZA - bobby digital
Snoop Dogg - Doggystyle
Snoop Dogg - Paid the Cost to be the Boss
Spearhead - Chocolate Superhighway
talib kweli - reflection eternal
Tribe Called Quest - Low end theory
Tribe Called Quest - Midnight
ultramagnetic mcs - critical beatdown
warren g - regulate… g-funk era
Wu-Tang - Enter the 36 Chambers
xzibit-restless