Not sure if I can really stand by this statement since I haven't played VI yet, but at least in terms of the most famous actions that the villain does in each game, VII remains more interesting imo.
Sure Kefka destroyed the world and poisoned a kingdom, but it's hard for me to really care about the lives of tons of civilians that aren't as fleshed out as the main characters. While the poisoning did affect one of the party members, said member isn't one of the three main heroes.
Compare that to Sephiroth burning down Cloud and Tifa's home town and killing Aerith. Just by themselves, they aren't anywhere close to being on the level of terror or devastation as that of Kefka, yet I still feel more affected by them. And that's because they're much more personal to the heroes, affecting how they ended up in the state they are now, and in the case of the latter, you grew so attached to Aerith and saw her interact with the others for such a long period of time that her death was one of the biggest gut punches in the entire gaming medium. Yes, FFVI also killed off a party member, but in VII's case, it was one of the love-interests of Cloud, and portrayed is as a tragic and senseless loss of life rather than a heroic sacrifice.
Just my thoughts. I might change my line of thinking once I finally get around to playing VI.