Freeza was overall better develop. And his legacy was cemented when we found out it was him that killed the Saiyans. He even went so far to kill the main characters best friend, and turned him into what he feared most. Even almost killed Gohanís adaptive father.
Freezaís name alone was synonymous throughout the Galaxy as the ruler of all. Before Super of course.
Where as Cell was a badass by killing Goku, he never had enough time to develop like Frieeza did. Cell did kill a lot of people, but none of them were as tragic as killing Krillin. Not even Trunks came as close.
Freeza was always far more chicky than Cell. Heís whiole persona screamed evil, especially for someone so short. Plus, he had bad ass underlings.
Freeza, definitely. I think it just comes down to him being fleshed-out more than pretty much any other villain.
I liked Cell more, though.
"A great tactician creates plans.
A good tactician recognizes the soundness of a plan presented to him.
A fair tactician must see the plan succeed before offering approval.
Those with no tactical ability at all may never understand or accept the plan.
And when a mind is too deficient in understanding, the resulting gap is often filled with resentment."
IMO, Frieza was the best personality-wise, while Cell was the best conceptually.
As for killing the Saiyans, that doesn't feel as important to Frieza now given how they integrated Beerus into the past where he apparently ordered Frieza to kill them, seeing them as a rowdy bunch...
Fighting-wise, Gohan vs Cell is still one of the most memorable parts of the series, and still the best looking Kamehameha clash (animation-wise) bar none.
Having hosted a tournament himself, can imagine how Cell would be criticizing the ToP's rules if he were there, although he would be legally able to fly according to said rules since he has wings I guess.