Sephiroth summoned meteor, either he lives or not, the meteor still hits ''btw, he said could defeat sin, didn't said he could defeat vuce versa, if Sephiroth tries to take it on with the masumune, then he's dead''
But it's the meteor that defeats it.
Sephiroth summoning Meteor shouldn't even count. He had to use the Black Materia in order to use Meteor, not out of his own power. Anybody could have probably done the same thing if he or she had the Black Materia.
Ultimecia lives in space, sin can't attack her, and she can do whatever she wants. She acheives her goal, all other existance denied, no more sin, and even in a 1vs1, she cast her spell and sin has 1 hp remaining, finishes sin off before he can attack. Maybe the same for Sephi.
BTW: I'm pretty sure that Eden GF would annihilate Sin.
Okay...

First off, Ultimecia does NOT live in space. She lives in a castle in the future. That's where all your characters went to go fight her in the end of the game. Secondly, it takes a long time for Time Compression to occur; my guess is over 1000 years. But before that 1000 years occurs, she only has power equivalent to any other sorceress. Her power is nothing compared to Sin and she would be annihilated. Thirdly, if you decide to include Hell's Judgement, you would be going by battle mechanics. And according to battle mechanics, Sin's Giga Graviton automatically kills anything. Ulti gets annihilated again.

By the way, there is no way to really tell whether Eden could annihilate Sin. After all, the only thing we have to judge Eden with is Eternal Breath, which doesn't actually destroy the galaxy.


And Also!
Sin Also needs to charge it's attack and takes a longer while than any other ff boss.
And as said before, it was because of the Hymn of the Faith and Jecht holding back Yu-Yevon that made Sin wait before using Giga Graviton.