I wish I knew what "momma's boy" really means...
Sephiroth is not a badass. His motivations are understandable, though. Wanting power is something that we can all relate to. Which is more than I can say for Kefka.

Kefka is like his opposite. Kefka was indeed a badass, although not a very interesting villain. Being insane for insanity's sake is not a very intriguing premise. While Seph wanted power and was prepared to destroy everyone standing in his way to get it, Kefka wanted to destroy everything and needed power to do it.

I killed him roughly 40 times or so to max out my level in Kingdom Hearts.  I got about 1 level per victory on him.  After the first like 7 or 8 fights, he gets really easy.

You can be a mammas boy and still want to take over the world like the badass he is, but there's nothing wrong with loving there momma, I admit I do

Badass

I beat him in KH1 but on LV 100 lol

Sephiroth wanted to destroy everything too...remember the black materia...which is what he needed inorder to destroy the planet...

*** major spoilers ahead ***

Kefka is more of a character than a generic madman.  Here is a summary of his character taken from wikipedia.

"Kefka Palazzo (ケフカ・パラッツォ Kefuka Parattso?, Cefca Palazzo in Japanese materials).[39] is a misanthropic madman and the game's central antagonist, dressed as a flamboyantly colorful Pedrolino and with a personality resembles that of a mad clown who enjoys destruction. Kefka serves as Emperor Gestahl's Court Mage and was the first volunteer for an experimental Magitek infusion, under the supervision of Cid. The process was still flawed, and although Kefka gained the ability to wield magic, it warped his mind and made him into the nihilistic psychopath he is during the course of the game.[40] He is also a rank coward, running from almost all in-game fights involving him, or employing Imperial Soldiers or illusions to fight for him."

Misanthropy is a general dislike, distrust, or hatred of the human species or a disposition to dislike and/or distrust other people's silent consensus about reality.

-- and --

Nihilism (from the Latin nihil, nothing) is the philosophical position that values do not exist. Most commonly, nihilism is presented in the form of existential nihilism which argues that life[1] is without meaning, purpose or intrinsic value.

So it seems that Kefka wanted to destroy the world because life is without meaning anyway.

I didn't get a good summary like that with Septhiroth.  However, from what I gathered, Sephiroth is convinced that he is a descendant from the Ancients and believes humans as traitors who abandoned the Cetra to a terrible disaster.  The Cetra (aka: Ancients) were betrayed by Jenova, but Septhiroth does not believe that.  Sephiroth then starts referring to Jenova as "Mother". A sort while later he is immersed in the lifestream where he ends up a the Great North Cave where he is located throughout the game (the other Sephiroth's were clones).  He devises a plan that if he were to impact the planet with a large meteor, the resulting gathering of the lifestream to heal the planet could be used to turn himself into a god.  I believe this is probably so he can exact his revenge on the human race.

It seems like to me that Kefka is the more interesting of the two.  But that is just me.  As to who the biggest badass is, Kefka is a coward, but Sephiroth just sits at the Great North Cave the entire game trying to control his clones and projecting images of himself.  My choice of badass goes to Chuck Norris.

Sephiroth's specific intent was to take the lifestream into himself and literally become the planet so that all living things would be a part of him.

Kefka's... very interesting, though, in that he acknowledges his own cruelty but doesn't seem to care. He's really kind of like every serial murderer in the universe come to life all at once.

And let's not mince words about nihilism, either. One can argue that Kefka was a nihilist, but that doesn't mean that he did not hold values, just that he held that no set of values was intrinsically correct. The only meaning in life, according to the nihilist perspective, is that which people create for themselves: Kefka rejected the meaning that other people expressed to him, and imposed his own values.

His own values just included being a gigantic psychopathic kid with a magnifying glass and a penchant for real and aware cruelty.

BadAss!!!!!

He's a momma's badass. Duh. :P