Quite obviously the answer to this is the nWo.
Not only did the initial nWo have two major superstars in former WWF Champion Kevin Nash (as Diesel) and former IC Champion Scott Hall (as Razor Ramon), but the impact of the biggest wrestling superstar and fan favourite of all time, Hulk Hogan, turning heel sent shockwaves through the wrestling world. It was genuinely a jaw-dropping moment. People never thought they would see Hogan as a bad guy.
The Nexus attack on John Cena was very surprising, and one of the best things the WWE had done in years but it was nowhere near the level of the Hogan heel turn. The fact the attack was carried out by rookies, not main eventers made it more shocking, but it cannot compete with the biggest face of all time turning heel.
Also, the fact is that the nWo was unique in the way it was carried out. Nexus were members of NXT, a WWE show, while the creative way the Outsiders invaded WCW was made to convince the fans that they were still WWF employees who were trying to ruin and takeover the rival companies show. This had never been done before, and was totally surprising.
The biggest factor I would say which makes the nWo the bigger faction, and I am only focusing on the original nWo here, not the Wolfpac, Black & White, nWo 2000 etc, is that Hogan became the heavyweight champion while in the nWo, Nash and Hall became tag champions, the nWo DID dominate WCW, while Nexus never managed to gather any gold, and Wade Barrett did not win the world title. They were shocking, and did cause a big moment by burying the Undertaker and getting Cena fired for 2 weeks, but they didnt win the big one, or really make much of a difference to WWE programming.
nWo dominated WCW, got their own PPV, their name on the WCW video game (WCW/NWO Thunder etc), they were just a much bigger deal. Plus they had the better music, better T-Shirt and the bigger name members
So the nWo is the better faction for me. By far