LOL
An Orochi vs. Sephiroth thread was made before and personally, I thought Orochi would have won that battle.
For some reason though, I think Jenova is above Sephiroth in terms of power...perhaps she would be a greater opponent.
Orochi is a godly powerful character. I personally think Orochi would win against Jenova.
We don't actually know enough about Jenova. All the fights with er in FF are just weak mutations
One assumes she was a being like Sephiroth and elite soilder of some sort, but we know jack about what kinda of soilder she was.
Though since mere particles turn a normal Human into a Sephiroth I'd say it's safe to assume, Jenova > Sephiroth easily