The best rap song ever

Branching off from today's list of the best songs ever, by genre, I noticed there was some controversy over what's the best rap song ever.

Songs mentioned so far:

• The Choice Is Yours, Black Sheep. My original choice. I wasn't that certain about it when I selected it, but it's definitely one of the all-time greats.
• Time 4 Sum Akshon, Redman. A pretty good song with a classic video. But better than the Black Sheep song?
• Jersey, Yo, Redman. Um, no.
• Luchini, Camp Lo. Good, but not on the same level as those first two.
• Welcome to the Terrordome, Public Enemy. Great, but if I was going to go with a PE song it would be Fight the Power.

So what is the best rap song ever, anyway? And does it strike anyone else as odd that there's no definite consensus now, like 25 years into recorded hip-hop?