is Sephiroth the best bad guy in the series?

[Goldenboko]

Kefka also has one unique claim to fame. Unlike any other Final Fantasy villain, such as Sephiroth or Kuja, Kefka succeeded in what he set out to do. In fact, he may be one of the only RPG villains in history that actually manages to succeed in his goals, destroying the world and becoming a god. It is his second, not his first, set of plans that the player ends up foiling.

Kuja succeeded in his first set of plans, Sephiroth suceeded in becoming a god in AC and Yu Yevon succeeded even before the adventure started.
And everyone know that, when we are talking about success, Delita wins.

If i want to, i can write in Wikipedia that Kefka is the most weak character in fiction.

And it will be fixed. Wikipedia has a very good moderating team, although not everyone knows that. And don't try and compare Seph's feats to Kefka's they don't compare because Sephiroth wanted to absorb the lifestream, didn't happen did it? Kefka wanted to destroy the world, and that did happen. The World of Ruin was basically a destroyed planet, life was fading rapidly.