Sephy cannot control the lifestream, therefore he did summon meteor, not only to destroy gaia, but uses north crater as a base and brings jenova there with him. This is important because jenova's presence in the crater is what makes the lifestream gather there so that sephy can control it (this is because jenova's fall/arrival on the planet is what created the North Crater in the first place). Basically he wanted to hold back the lifestream like he was doing to Holy. That way meteor (which is what he was doing with his own hands) could not be stopped.

Going back to the original topic of the thread though, I actually believe that the greatest villain (greatest in terms of most evil that is) would be Ultimecia from VIII. seriously though, whats more evil than a powerful sorceress who isn't content with conquering and controlling the world, but goes further to project her evil spirit into the past in an attempt to merge the two time periods of the world and control it as well?