First, you have to answer what do you consider WCW....some people look at WCW as a completely different entity than the NWA, they only consider WCW post 1989 when Turner basically bought all the interests of the NWA product that ran successfully for years from Jim Crockett Jr. Other people consider WCW as going back into the NWA, essentially seeing it as the same company, just changing owners.
If you only count from 1989 on then it's pretty much a two horse race between Flair and Sting. Sting was one guy who never left, unlike Flair and Luger, both of which bolted in the early 90s and had successful runs in WWE. During the really dark days of WCW with Flair gone, Sting almost single handedly kept the brand afloat with his feuds vs Rick Rude and Vader, as well as Luger. Luger may have been chosen to be the first long term champ post Flair's exit but Sting was the centerpiece, much like John Cena today.
Once Flair returned the product was invigorated. Older fans who had tuned out slowly started tuning back in and the numbers started ticking significantly upward as Flair moved back into active wrestling, then into the main events with his well received feud vs Vader. Flair again was carrying the company, all over TV, doing house shows, and wrestling some very good matches vs Vader, Paul Orndorf, Steve Regal, Rude, and others. Lets not forget that behind the scenes Flair was instrumental in getting Hogan to sign, using their friendship from WWE and offering to put him over in a three match set and make him champ. Regardless of your opinion of Hogan regarding his backstage politicking and self promotion, there is no doubt that the numbers ticked up further when he arrived, at least when he faced Flair. Flair also played a big role behind the scenes in recruiting Randy Savage.
Flair really carried the weight through 1996 as the most consistent top star, the company's lead villain, and he was the featured star in the early days of Monday Nitro dominating the World Title from late 95-Spring of 96. Injuries and backstage conflict with Hogan & Eric Bischoff lessened his role somewhat post 1997 but even during 97-99 Flair had prominent fueds with Curt Henning, Sean Waltman, wrestled two phenomenal bouts vs Brett Hart, was the 2nd biggest attraction at Starrcade 98 behind the Goldberg-Nash bout, and delivered big numbers again for his final run vs Hogan in 99 which essentially ended the Now storyline as well as gave Flair another World Title Run.
I would actually give Sting the edge though from 97 on as work horse or face of the franchise, he was even more prominent than Flair or any other WCW star opposing the NwO. No doubt that DDP became a big star and Goldberg was a huge phenom, bigger than anyone in WWE except Austin, but those werev shorter periods and less substantial in terms of storyline and match placement over all as compared to Sting.
So again, 1989 onward, it's pretty even between Sting and Flair and for longevity sake Id probably give the nod to Flair if I had to pick only one. If you extend this back and consider WCW just an extension of the Jim Crockett Promotions and the NWA then it's Flair by a mile, at least post 1980.
