Cloud and Aerith were Sephiroth's greatest obsticles in summoning Meteor and therefore, those were his two main targets; Cloud was a constant and Aerith suddenly came up.
So there, Sephiroth takes out one target and has one left to go. But why not kill two birds with one stone and get rid of one target and majorly devistate the other to the point where the remaining target barely has the will to go on. Sephiroth more than liekly new the destruction it would cause Cloud to lose Aerith regardless of how it happened but Sephiroth, being the mad man that he is, decided to wait for Cloud to find her and then bring the devistating blow and kill Aerith in front of him. He didn't so much kill her as ALLOWED her to live up to a certain point. What he didn't time correctly, I suppose, is her summoning Holy. But once he saw that she was alone, it would've been to easy to kill her and have Cloud find her dead at the altar in the Forgotten City.
So he let Cloud chase after her. Sephiroth practically used the opportunity to bait Cloud, having him chase the one he cherished most in hopes that he'd find her. He let Cloud find her and speak with her one last time. He gave let Cloud believe that everything was going to be okay up until the VERY last moment, the tiniest bit he could allow. And the suddenly, when Cloud would think that no harm has been done and Aerith can safely return to him, Sephiroth swoops down from above and takes all Cloud holds dear. How's that for devistating? He played Cloud like a fool and really let him have it where it hurts the most.
He taunts him, telling him that Aerith will become part of the Planet and even then she will be in pain once Meteor strikes. Sephiroth continues to tease and torture him with the fact that her soul will continue to suffer even though she's dead. Like Angel Alex mentions, he binds Holy, Aerith herself, to prevent what she set out to do. He mind controlled Cloud to beat Aerith at the Temple of the Ancients, he came into Cloud's dream to show him, "Yes, I'm going after her. Yes, I will take her away." Thus causing Cloud to spiral into a frenzied panic to find Aerith as soon as possible.
Not once, not ONCE do I remember Sephiroth going to such lengths to bring such pain and torture to Cloud by using Tifa. You could say he tried to kill her during the Niblehiem incident but that was only because she was brazen enough to try to take him on herself. Sephiroth, I'm sure, was well aware of the importance of Aerith to Cloud's life and by using her to bring pain to him, in a way, Aerith was the ultimate weapon against Cloud BECAUSE he cared so much for her. Sephiroth was using Cloud to hurt was he loved most and because of that, Cloud was being torn apart. Sephiroth would've probably continued that had he not discovered Aerith's plans to summon Holy. So he got rid of her AND damanged Cloud both permanently and devistatingly. By killing Aerith in front of Cloud, Sephiroth brought Cloud to his knees. He devistated him in the present and that one night is what continues to haunt Cloud. Sephiroth gave him the ultimate wound that would continue to haunt Cloud forever because even though he realizes he's not alone at the end of AC, Sephiroth, always being present in his mind even slightly, can continue to taunt him with her death almost saying, "You couldn't do a thing."
In AC, this proves to be true because when the question of what is most precious to Cloud comes up, Aerith comes to mind immediately. Yes, Tifa and the kids come up later, but Aerith was first in his mind, because she was the most precious thing that Sephiroth took away. Sephiroth took his home, his mother and his best friend. But of all those things, the most precious of them all, the one that holds the most value to Cloud's heart was Aerith. Tifa and the kids are what COULD be taken away. In a sense I viewed that scene as, yes, Tifa and the kids are extremely precious to Cloud and they could be taken away because they're still there, alive and in danger. So Cloud fights. But Cloud first thinks of Aerith and the way I viewed it was, Sephiroth's already taken her away, the most important person to him, what more could he lose? And then that's when the thought of Tifa and the children come up, because he could lose them too. Besides, if they wanted to illustrate Aerith as being important, then WHY would they show her three times? I'm just saying that on SE's part, that was bold move.
Cloud promised to protect Aerith and knowing that she died while under his care, knowing that he had to protect her because it was his duty and later, because he felt he had to because he grew to love her, is what would hurt the most. Sephiroth is a very intelligent man because he knew here to hit Cloud the hardest. And he got it right on the nose. Not once did I see him focus that much attention on Tifa. Why go to all the trouble of getting rid of Aerith if he didn't believe that Cloud cherished her so much? Sephiroth's a smart cookie; if I were him, I would've done the same thing because no other event, I believe, that's been shown to us has caused Cloud that amount of pain.
And that, ladies and gents, is why I think Sephiroth was Cleris and he used that to his advantage. I think everyone was aware of the affection growing between Cloud and Aerith during the time that she was alive, hence why a few of them, after giving their respects to Aerith, tried to comfort Cloud at the time of her death. They more than likely knew the saddness he felt at losing someone who, probably very obviously to them, he cared for as more than a friend.