5 - La Familia: I'm probably the only person in the world who will mention this group, and I don't care. La Familia accomplished something NO other faction in wrestling history did, having two of their members hold recognized World Championships simultaneously. That's a damn impressive feat.
4 - The Power Trip: Although other groups had dominated their respective companies, never before had TWO men done it so effectively. Stone Cold was WWE Champion, Triple H was Intercontinental Champion, and they won the World Tag Team Championships together as well. The Power Trip took full advantage of the amazing heel turn Austin had made at WrestleMania, and these few months were the best run of Austin's career in my book. Once Triple H got injured, Austin was quickly turned face and reverted to being the same character he'd been for years on end. Although they turned him heel again later for another few months, it just wasn't the same. If Triple H hadn't gotten hurt, this might have been the best faction of all time.
3 - nWo: The original version of the nWo made WCW the hottest wrestling product on the planet, and with damn good reason. If the concept hadn't become so watered-down due to new members joining practically every week, WCW may very well have won the Monday Night Wars. And on a side note, I don't care what anybody says - DX was nothing but a piss-poor ripoff of the nWo.
2 - The Four Horsemen: Easily the most iconic faction up to that point, and one of the earliest to make such a huge impact. With Ric Flair as NWA World Heavyweight Champion, Tully Blanchard as NWA United States Champion, and The Andersons as NWA Tag Team Champions, the Horsemen completely dominated NWA wrestling for such a long time that it seemed like the group would be on top forever.
1 - Evolution: I personally prefer Evolution over The Four Horseman, since I prefer the Triple H and Randy Orton over any of the Horsemen other than Flair. While Evolution was undoubtedly a rehash and homage to the Horsemen, I feel Evolution made more of an impact in a shorter time.