That we see? Everyone in Nibelheim (except for Cloud and Tifa and Zack and ... OK, not everyone everyone). The president of Shin-Ra.
Beyond that, I don't think we see Sephiroth ever kill anyone really specific except various unnamed Sephiroth clones.
In fact, his goals don't really involve killing people except as a side effect of trying to destroy the planet.
He is shown to kill most Niblheim residents, the Midgar Zolom, President Shin-ra, and Aerith, but he is implied to kill Wutai troops during the war.