As Walk-In says, it depends on what criteria you're going by. Pure stars, who put on the best matches, who was the most invididually talented etc. I'll just name-drop my top 5 in terms of matches, performances and all round ability as a wrestler/character. It'll mainly be from the NWA era into 92-93 WCW:
5. Steamboat/Eddie Guerrero
4. Vader
3. Arn Anderson
2. Dustin Rhodes
1. Ric Flair
Eddie's 97 is one of the finest individual years for a US wrestler I've seen and he looked like the undisputed best in the world on a weekly basis. His performance in the Fallbrawl 97 match vs Jericho might be one of his finest individual performances ever and its amazing how much better than Jericho he is in the match, to the point where you could have replaced Jericho with any half decent babyface and the match wouldn't have suffered Eddie was that good.
Steamboat from 89-94 just had an incredible run. The Flair series in 89, the Funk and Luger matches from the same year, the 1991 tag w/Dustin vs Arn and Larry, tag with Eddie Gilbert vs Flair and Luger, the Dangerous Alliance feud and the subsequent Rude, Austin and Eaton singles matches as well as the numerous tags. The Austin US championship feud as well as the Regal matches in 94, the Vader bouts including the excellent Human Cage encounter etc.
Vader...well enough good words can't be said about him. One of if not the best big man worker of all time. His 1993 year is a career year where he seriously looks above and beyond anyone he wrestles, be it Davey Boy, Ron Simmons, Sting, Flair or Steamboat. He's the perfect foil for all of them and is so astute at laying out the matches to give enough to his opponent for them to get over as a threat whilst he remains a killer on offence. The Sting series is obviously what most remember him for as well as the Flair Starrcade 1993 match, but I actually find his Dustin Rhodes matches from 1992 and 1994 to be some of his most underappreciated work for how incredibly awesome they are. They're really not far away from making me think Rhodes/Vader should be the blueprint of David vs Goliath rather than Sting/Vader.
Arn...amazing worker whether he was being the ultimate comedy stooge or a merciless machine on offence. So astute and entertaining in a variety of settings against a multitude of different opponents, whether it was his TV Title run, the Dangerous Alliance run, fighting Flair in 1995, the Doom brawls in 1990, Flair/Arn vs Hollywood Blondes etc. Dude was relentlessly entertaining in the ring and on the mic, 1992 is a career year for him where he looks to be the front-runner for best wrestler in the world.
Dustin might be an 'out there' pick to some, but his catalogue of matches is insane. Multitude of tags in the Dangerous Alliance feud, takes part in the GOAT WarGames match in 1992 and follows that up with another sterling effort in the 1994 Wargames, Bunkhouse Buck feud as well as the on and off Vader feud are super angles in their own right and he looks terrific in a variety of settings working against Windham, Larry Z, Arn, Pillman, Eaton and numerous others.
Flair...no explanation needed really. His 89 run speaks for itself with the Steamboat and Funk feuds, his matches with Sting helped establish Sting as the impending face of WCW, terrific run in 1993 with the Vader matches as well as working the Blondes, has a great 1994 with the two Hogan matches in particular being far better than they have any right to be, christ even in 1996 the dude is producing greatness whilst working the likes of Eddie and Big Show, and the Flair/Arn vs Savage/Hogan Nitro 1996 tag is too overlooked when it comes to great WCW tags.