WCW was on absolute top throughout 1997. The New World Order was a mainstream hit, Flair and the Horsemen were still around in some capacity, the under-card workers were consistently putting on matches that eclipsed anything else on a national scale, and most of the angles still made sense.
1998 was a strong year for them, but they went overboard with the celebrity appearances. Goldberg's rise to fame came to pass, but it was tainted by all the illogical turns, swerves, and other bullshit that was going on - Hall turned on Nash, Giant flopped back and forth too many times, Sting and Luger (the top WCW defenders from day one) became members of the Wolfpac.
Ugh. 1998 had its moments. Raven's Flock, DDP's steady climb to the top, Benoit's recognition as one of the greatest, and Jericho's unmatched heel run all make 1998 a viable candidate also.
Both were REALLY strong years for business. WCW was doing big box office business up until the spring of '99.
As far as ratings and popularity I'd go Mid-96/97/98. Product started to dwindle in 97/98, but that was really nothing into comparison of how shit it would become in 99/00.
My personal favorite WCW days were before it was actually WCW (JCP), World Championship Wrestling was just the name of the TV program on TBS. No I wasn't alive, but I've seen the majority PPVs and a lot of the TV programming through WWE 24/7 and it's got to be the late mid-late 80's before Jim Herd took it over. When Turner did buy it 1988 and 1989 were also amazing year as far as the actual product goes. The in-ring product and the feuds were just so much better. The cards for Starrcade shat on early WM cards from a match quality standpoint as well.