Just one?
Hoo Boy.
Depends on what kind of rap you’re talking about.
I’d say the greatest gansta rap anthem was probably Geto Boyz - Mind Playing Tricks On Me.
Party Rap - fer dancin’ - DJ Kool’s Let Me Clear My Throat
Song Praising Women’s Derrieres - Rumpshaker, by Wrecks N Effect
Greatest MC - I know Biggie and Tupac have their fans, but I’m a Rakim defender from waaaaay back.
“Ain’t No Fun (If The Homies Can’t Have None” - by Snoop Dogg with Korrupt and Warren G
Maybe not the “best” per se, but the one I like the best, simply for its audacity.
Sample lyric:
"Guess who’s back in the muthafuckin’ house,
with a big fat dick for ya muthafuckin mouth?
Hoes recognize, bitches do too,
‘Cause when bitches get scanless and full of voodoo
What you gonna do?
You really don’t know
So I’d advise you not to trust that ho
Silly of me to fall in love with a bitch,
Knowin’ damn well I’m too caught up with my grip
Now, as the sun rotates and my game grow bigger,
how many bitches want to fuck this nigga named Snoop?
But in terms of skillful rapping (plus an absolute knockout video), I’m voting for Busta Rhymes’s duet with Janet jackson on “What’s It Gonna Be?”
I hated 99.9% of all the rap I ever heard, but I do have a couple songs I like:
Geto Boyz - Damn, It Feels Good to be a Gangster (from the soundtrack of one of the funniest movies ever - Office Space.)
Bone Thugs 'n Harmony - just about all of their East 1999 Eternal CD. Amazing. How do they rap so damn fast?
Another hip hop artist I admire for his vocal abilities is Rahzel. He beatboxes (makes very realistic bass sounds with his voice) and sings or raps at the same time. Flawlessly, too. I just don’t know how he does it.
I liked “Rapper’s Delight” as well, but the one I like the most is Coolio’s song in the movie where Michelle Pfieffer teaches kids (darn, I just had my mind go blank). The movie was based on the book, “My Posse Don’t do Homework”. I hope someone out there knows which song I mean. I think Coolio was nominated for an Oscar for that song.
That would be Gangster’s Paradise, medstar.
I’m rather partial to Bust A Move by Young MC, myself.
White Lines- Grandmaster Flash
Sabotage- Beasties
and just for fun: Baby’s Got Back
(I like “Bust A Move” too:))
No question - Those listed so far don’t hold a lyrical candle to these two.
1: ODB’s Hippa to the Hoppa - so many levels to that song
2: Bone Thugs-N-Harmony’s Thug Luv - sheer brilliance
Fight The Power - Public Enemy
Bring Tha Noize - Public Enemy w/ Anthrax
It Was a Good Day - Ice Cube
Nothing But A G Thang - Dr Dre
Hail Mary - Tupac
Dear Mama - Tupac
My pick is ** Caught Out There ** by Kelis " I hate you so much right now!" Although like The Outkast she might be considered more hip-hop by some than rap.
Athough, like Weird_AL_Einstein, I did truly like Snow a great deal. At least the 1st album, anyway… 
Judge my tastes in rap by that as you will.
Yes of course Bust A Move was very good, as were several others by Young MC the names of which I can’t recall off the top of my head. Also Wild Thing and Funky Cold Medina by Ton Loc.
I also agree about Gangsta’s Paradise, and of course cannot let the opportunity to mention Weird Al’s parody of it, Amish Paradise, just slip by :D.
the Geto Boyz “Scarface”. A whole urban gangster flick encapsulated in one song.
Follow the Leader - Eric B & Rakim
would you all consider Come with me by Puff Daddy and Jimmy Page rap? If so, then my favorite rap song is that.
Runners up would be
Bounce by Run DMC
California Love by Tupak
I’ll second, third, etc. Sugarhill Gang- Rappers Delight, Grandmaster Flash- The Message, Snoop Dogg - Gin + Juice, and Dre- Nothin’ But A G Thang.
And add, Herbie Hancock- Rockit and Bizmarkie- Just a Friend. “Rockit” is possibly the most sampled track in Hip Hop and even though “Just a Friend” wasn’t especially influential it is damn funny.
But my personal favorite of the OG’s has to be:
Eric B. and Rakim- Paid in Full
“Thinkin of a master plan
Cuz ain’t nuthin but sweat inside my hand
So I dig into my pocket, all my money is spent
So I dig deeper but still comin up with lint
So I start my mission- leave my residence
Thinkin how could I get some dead presidents
I need money, I used to be a stick-up kid
So I think of all the devious things I did
I used to roll up, this is a hold up, ain’t nuthin funny
Stop smiling, be still, don’t nuthin move but the money
But now I learned to earn cos I’m righteous
I feel great! so maybe I might just
Search for a 9 to 5, if I strive
Then maybe I’ll stay alive
So I walk up the street whistlin this
Feelin out of place cos, man, do I miss
A pen and a paper, a stereo, a tape of
Me and Eric B, and a nice big plate of
Fish, which is my favorite dish
But without no money it’s still a wish
Cos I don’t like to dream about gettin paid
So I dig into the books of the rhymes that I made
To now test to see if I got pull
Hit the studio, cos I’m paid in full”
Also…a little trivia for ya… Rapture is the first rap song to break the top ten…
SOOO…the first rap song was by a white chick…lol
You know, I always thought that Teena Marie’s Square Biz came out before Blondie’s Rapture.
Rapture had better shout-outs, but Square Biz clearly had the better lyrical flow – and did materializtic bragging loooong before these others got in the game.
Heh, I am listening to Prefuse 73, and this is pretty good, but I don’t think it’s old enough to be classic and it probably won’t ever be well known enough. 
I’d say “911 is a joke” is the best.
honorable mentions
Humpty Dance
The Message
White Lines
ANYTHING ELSE by Public Enemy
No Sleep til Brooklyn.
Mama Said Knock You Out
I think Notorious BIG is so overrated it’s incredible. i haven’t heard a single thing by him taht impressed me, I don’t think his lyrics are very good, I think the fat strangled his vocal chords so his voice isn’t interesting.
Fuck tha Police; I gotta mention this one. It’s an influential song, and classic for sure, but I’m gonna have to eliminate it from the running for myself due to the fact that Dr Dre followed up Ice Cube’s really eloquent social commentary with a bumbling attempt at the same, and then Easy E, made the song sound like the police are right to beat people like him down because he really is a menace, ruining every point Easy E made and bringing Dre down instead of Easy E bringing him up.
Erek
I like rap and stuff, but honestly, I can’t tell the difference between hip-hop and rap…but here’s my favorite “rappish” songs…
Grouch & Zion 1- Silly Puddy
Grouch - Rap is Senseless
Grouch - Simple Man
Grouch & Sunspot - Loose Cannon
Grouch - Grouch’s Prayer
Murs - Rules The World
Pharcyde - Can’t Keep Running Away
Lauryn Hill feat. Nas - If I Rule the World
Benefit - So Sick
I’m suprised my favorites have not been mentioned yet:
Jump Around by House of Pain
Insane in the Membrane by Cypress Hill
Rough Riders Anthem by DMX