In terms of:

Strength: Sephiroth

Looks: Tie, I say!!

Endurance: Ansem, easy

You beat him once, he creates that huge dome and fights you again. Twice, he brings you into Kingdom Hearts itself. Thrice, he goes into his Guardian and awaits your defeat by his grand army of Heartless as he regains strength. Four times, Ansem opens the Door to Darkness and is overcome by the Light within. A little shaken, yes, but he remains as though he is yet to be harmed as he opens the Dark Door. In KH:CoM, Ansem returns and troubles Riku ever so relentlessly. Once defeated, he proclaims he shall return. Since Billy Zane is listed as doing the voices of Ansem/DiZ, it is obvious he is returning. I think it would be awesome to fight Ansem in the final battle after obtaining the power of Kingdom Hearts.

So, ffxfreak93:
Ansem=Sephiroth