No doubt the Four Horsemen. For one, they put stables on the map. Sure there were groups before them but their success helped make them great. The original 4 (Flair, Blanchard Ole and Arn) were the most devastating. Ole and Arn should have had a run with the World Tag Team Titles. But the Horsemen (Flair, Windham, Blanchard, Arn) was probably the best.
Now the NWO angle was fun to watch but they made a lot of mistakes. Probably got to big. If it would have stayed between 4 and 6 guys, it could have worked better I think.