There have been quite a few Stables in wrestling history from the J.O.B Squad to Evolution.

Who has been the best?

You could consider things like longevity of the group
If they held any championships
Legacy they've left behind
Whether or not they produced stars after disbanding

The 4 Horsemen obviously has to be at the top or very much near it.
nWo before it got inflated. July 96 to the end of 97/begining on 98
Because I'm very much bias I'm going to also say Nexus

(I'm just going to make the the rule that it has to be 3 or more members for this)

The Four Horsemen.

the nWo. The longevity was a problem, the amount of stars and how far they took it, was a problem. But the original concept & the original design, was genius. And for a few years was the top stable in pro wrestling. It produced Championship, it was made of top stars and had some more stars added, regardless of how they did and how long they did it.

Four Horsemen definitely

On paper (bc I never got to see them actually fight together) the Original Legion of Doom: Paul Ellering, the Road Warriors, King Kong Bundy, Jake Roberts, Matt Bourne (Doink), the Iron Sheik, the Original Sheik, Arn Anderson, and the Spoiler.

of the folks I've actually gotten to watch:
1. Four Horsemen
2. Triple Threat
3. nWo
4. Fortune
5. No Remorse Corps

Not saying they're the best, but they deserve a mention... The Heenan Family.

Can't believe i forgot it before....BDK.

The Horsemen.

Evolution. Made a world champion out of Batista (crazily) and Orton (later on)

NWO and The Hart Foundation

Gotta say that the Original Legion Of Doom sound bloody awesome!

My personal favs are The Heenan Family, The Hart Foundation and The Nation Of Domination! Oh and a mention to the J.O.B Squad!