Rapper's love to diss each other and there have been many classic records made for just that purpose. It'd be a good idea to list them, with a bit of background information. That way old fuc kers like me can keep track!!
My nominations would have to be:
"The Bridge Is Over" by Boogie Down Productions
"Ether" by Nas
"Takeover" by Jay-Z
"Bitch In You" by Common
"Pimp Slapp'd" by Snoop Dogg
There's loads more I know but I've gotta plough through my collection to find 'em.