i was watching the episode where Cell was describing his hunt for 'Perfection' to Gohan.
it came to the bit where he was describing his own cells. as we all know that saijans become much stronger after recovering from a near death experience, si i thought that if Cell on-purposly made the Z-Fighters beat him to within an inch of his life, when he recovers he can just become more and more stronger.
So basically, he could sorta be beated almost to death, recover himseldf and become stronger, and just repeat it over and over, thus making him the strongest EVER !!!