I cant pick one, so I'll just throw a few out there.
Eazy-E - Real Muthaphukkin Gz (Deathrow diss)
Nas - Ether (Jay-Z diss)
Evidence - Search 4 Bobby Fisher (Eminem diss)
Twista/Speedknot/Newsense - Crook County (Bone diss)
Bone Thugs - U Aint Bone (Three 6 Mafia, CC, Do or Die diss)