Like others have said, that's hugely subjective. I did think the dream matches in this series (KOF '98 and 2002) were definitely among the most popular. But everyone has specific tastes, and others like other games in the series over these.
KOF '98 was an awesome game simply because it was a dream match. The only minor problems I had with it was that it had Rugal again (reapprearing after KOF '94 and '95), and you can only fight 6 teams to get to him. I wish we could fight random edit teams, and every single team to get to the bosses like in KOF '96 and '97 to extend the life of the game, I think that would've made for some interesting matches that would've made this game more enjoyable to play; it's almost like playing 2 player against the computer since we always mix and match our characters up to make up our team. The way things were in KOF '98 1P mode, they were all set teams you always had to fight against and that could get repetitive after a while. Other than that, I liked this game simply because it was a dream match with so many characters from previous games combined, it made for great fun.
KOF 2002 was very much the same. The difference here was the update in graphics which I think still couldn't compete with the awesome interactive backgrounds of the older games of the KOF series. Overall, I've always thought KOF '98 and others was funner to play than this one, mostly because the bosses and characters in general have become alot cheaper, but this game had other added advantages like fighting the newer teams after KOF '98 with the older teams combined, that made for some interesting matchups we could only dream about before KOF 2002 was around. It's a good game.
My conclusion to all this, if you really want to know what is the "best" game in the series, you'll have to play every KOF game to find out which one you liked best. Each were different in their own special way.