The Horsemen deserve to be in the conversation, in fact they were the best. ANyone watching wrestling between 85-88 knew that throughout the NWA the Horsemen in their heyday were as big and over as the NWO. The Horsemen were the first true bad ass heel clique that actually got cheered from fans, and one of the few cliques with multiple members being capable of drawing in the main event spot, not just one top guy and a supporting cast.
Also, the NWO really only had a two year run from 96-early 98. By mid 98 there really wasnt an NWO anymore, there were two competing subdivisions, a Black & White Group that was mainly heels consisting almost entirely of mid carders like Virgil and Scott Norton, occassionally Scott Hal and Curt Henning would show up but only for short periods (both had plenty of personal issues to keep them out off the show), Hogan was there for parts, but he wasnt fronting the group anymore he was entirely in his own feuds. The Wolf Pac was a fan fav group with former NwO'er Kevin Nash and a revolving door of WCW Fan Favs like Sting, Luger, and Brett Hart who were niether heels nor part of the NWO Invasion angle, neither stable was really strong although the Wold Pac got more play, in part because the WCW guys were able to be on screen anymore. What it wasnt was the NWO, with most of the original or founding members, and it wasnt very good. By the end of 1998 WCW's two biggest PPVs had almost no NWO connection at all, Hogan's feud vs Ultimate Warrior was a 1990 WWE re do that really didnt involve anyone other than those two (unlike when Hogan was challenged in the past by the likes of Sting, Flair, Piper, Luger, and the whole NWO was involved and covered his back, just like The Horsemen with Flair in the 80s), Goldberg vs DDP & Goldberg vs Nash were in no way promoted as having anything to do with the NWO. Only Flair vs Bischoff at Starrcade really was pumped up as an NWO vs WCW thing in the company's last two big shows of the year.
The NWO did have a mini revival in 1999, reuniting Hogan, Hall, & Nash, and purging any mid carders (Disco Inferno was an NWO Intern for heaven sake!!!) and re establising the unit as lean, tactical, killer heel group. It was short lived howver and by March of 99 the NWO broke up and essentially was done. There were mini revivals like The Jeff Jarret era in 2000 and the extremely brief WWE Invasion in 2002 but they were never a major force for a prolonged time after that. You cant even argue that the NWO was on top for four years because the whole story and the group itself basically changed and ceased to exist for much of 1998 and the 99 revival lasted less than four months, other incarnations were too short and not very successful. Over a four year period The Horsemen 85-88 had a greater impact than the NWO 96-99, largely because 98 was a cluster $#@% and 99 was way too short (I actually really liked the whole Nash heel turn, Finger Poke of Doom, etc, I feel WCW realy blew it not focussing on an all out NWO PT 2 vs Flair & Goldberg, especially considering they had Sting & Brett Hart on the shelve injured, their mid year returns would have huge impact, but it's all water under the bridge now).
DX really was a couple of alternating groups with the constant being HBK & HHH. Now you can say that the core of the NWO was Hogan, Hall & Nash but the fourth "major member" changed a bit (Big Show for awhile, Savage for a bit, Henning for a bit) with Scott Steiner evenually settling in and giving the group a consistently good four man front. DX never really had that, although for awile the NOA were a pretty hot tag team. Even The Horsemen had three core guys (Flair, Blanchard, & Anderson) while rotating the fourth spot (Ole, Luger, Whyndam). Essentialy DX was HBK & HHH's tag team with other guys coming in for a bit. Also, unlike The Horsemen & The NWO, DX at their height was not the centerpiece of WWE programming. WWE 1998-99 had a lot of Faction Warfare and several main event or near main event level feuds that involved the likes of Austin, Rock, Foley, & Taker but not DX. In fact, HBK wasnt even a factor after WM in 1998, although the group lived on for a bit with HHH as the anchor before he was split off on his own. Almost every major title and feud in the NWA from 1985-88 involved some incarnation of The Horsemen... Dusty vs Flair & Magnum vs Blanchard in 85, Dusty's tilts vs Blanchard in 85, 86, & 87, some of that time Magnum feuding with Flair. The Road Warriors vs Flair and at other times Blanchard & Anderson, Koloff vs Flair in 86 and Luger in 87, Whyndam vs Flair in 87, The Horsemen vs The Garvins in 87 (Both Flair and Blanchard had major feuds vs Ronnie Garvin in 86-87), Sting feuding with both Flair and Blanchard & Anderson in 88, Luger vs Dusty, and later after leaving the group Luger having multiple runs vs Flair and The Blanchard/Anderson tandem in 88. Certainly when The NWO was at its best they were the cenerpiece of WCW programming, the idea of the Invasion, taking over the company and bleeding it dry, the idea of former WCW rivals joining together to fight off their common enemy, pretty much evey major WCW angle during the year & half from summer 96 to early 98 when they split the group and members started disappearing involved some combination of WCW biggest stars like Sting, Flair, Luger, DDP, and at times Savage vs some combination of NWO guys. At best DX was a part of an ensemble that was dominated by Austin & Rock and who they battled. They had their moments but too many times the path to the main event did not end with them. That wasnt the case with The Horsemen or the original NWO.
Now all of these groups had incarnations after their heyday. I dont think anyone looks at Jeff Jarret's NWO in 2000 as a legendary ground breaking heel faction. Likewise The Horsemen with Sid Justice or Paul Roma is embarassing. DX decided not too even bother with other members, to just focus on HHH & HBK like no one else ever existed, which was OK but their sophmoric humor came off really bad during their mid 2000's revival (it's hard to be taken seriously as pransker teenager character when you're in your mid 40s). HHH had spent too much time as a super serious, Flair Like Villain fronting Evolution to go back to the juvenile antics of old DX, HBK had spent much of his time in his come back being portrayed as the company's resident elder statesmen legend, he looked just as sad trying to recreate the ridiculousness of original DX. Perhaps that is the biggest handicap to DX in this discussion, for most of their existance they were as much about comedy as they were about being serious villains, they were not the cut throat, cunning rulebreakers The Horsemen were or the dangerous, gang land mentality rutheless aggressors the original NWO were. They were potty jokes, comedy skits beating up homeless people, they just were not serious enough.
For me it comes down Horsemen 85-88 vs NWO 96-early 98/early 99 - WCW's national presence was huge in the late 90 but The Horsemen dominated the storylines and big matches longer and definately were better in the ring and on the mic than the NWO. The NWO Invasion Storylne may have been the biggest storyline in wrestling in the decade of the 90s but The Horsemen were a better group. DX wasnt as serious, they didnt dominate the top of the card as much as the other two in their glory periods, but they were great on the mic, better than the NWO in the ring, HBK putting over Austin at WM was a defining moment for WWE. They were good, just not quite as good as the other two.