The best diss song ever released is No Vaseline by Ice Cube. Obviously. None of this Eminem buisiness can touch that.

The kings of diss:

The Bloodhound Gang.

Let's see
-Everlast's eminem diss...i forget the name...but em gets what's commin to him.
-Eminem - Can I Bitch...oh shit it's jermaine propaine...i was laughing contently in my seat until eminem brought Jermaine Dupree into the picture...then i was damn near crying.
-Insane Clown Posse - Ain't nothin but a bitch thang...Love or hate the clowns...it has a terribly original beat and it just kills anything Eminem ever said about them...he hasn't responded since.
-Pink Floyd - Dogs...They're pretty much dissing corporate america in this masterpiece...so much bite yet so melodic.

NIN - No You Don't (Courtney Love/Marilyn Manson diss)

NIN - Big Man with a Gun (gangsta rap in general diss)

Slipknot - Get This (west coast metal scene diss)

Clutch - Careful with That Mic (Eminem diss, and by far some of the most intelligent and flamboyant rapping I've ever heard)

Kiss Off by the Violent Femmes :)

Whitey's Revenge

Definitely a hot track