Sephiroth is apparently by far the most powerful despite being the child of a random and a scientist and despite others being mentioned getting cells (I beleive, if not mako), by the end of the game, Sephiroth > Jenova. Stupid weak evil alien of doom! (Says the game)
Edouble explained why Sephiroth was the most powerful result of the Jenova project. I'd like to add that all this "Sephiroth versus Jenova" stuff everybody's always harping about is nonsense. Sephiroth essentially
is Jenova, manifested in human form. Their motives and actions are the same. Look to the concept of the trinity in Christian mythology, which this relationship was probably based on (hence Jenova -> Jehova), for an analogy.
Cloud is a normal guy who wants to be big and tough. (Says his Childhood)
He gets killed by Sephiroth who plummets to his death as well with cloud IMPALED ON HIS SWORD. (Says Cutscene as he impales cloud and cloud plunges them both into giant death pool of mako)
His friends think he has amnesia and are afraid to tell him the truth because it will destroy him. (Says Tifa)
Somewhere in this story, both of them re-emerge as clones.
I think you may be the one with amnesia. Cloud wasn't killed by Sephiroth, and he didn't fall into the lifestream with him. He was stabbed through the shoulder before overpowering Sephiroth and tossing him into the lifestream. He was then found by Hojo on the catwalk and entered into the Jenova project along with Zack, where they were treated with Jenova cells and Mako energy.
Neither Cloud nor Sephiroth are clones of anyone, or at least not in the sense we think of clones. Sephiroth didn't die when he fell into the lifestream: his body became infused with the knowledge of the ancients and it drifted to the northern crater, where it remained until the final battle of the game. The manifestations of Sephiroth encountered through the rest of the game were various parts of Jenova's body made to look like Sephiroth through Jenova's power of illusion. As for Cloud, he was a clone only in Hojo's strange definition: "Sephiroth clones" are just ordinary people who have been treated with Jenova cells/mako in an attempt to duplicate Sephiroth's impressive powers.
Now, does anyone have any criticisms of FFVII's story that aren't based on misconceptions/stupidity?