Based on current known evidence? Broly. Theoretically? Cell. He has the DNA of Frieza, King Cold, the Saiyan's, Piccolo, and the humans.
The humans alone can overcome pretty massive gaps in power if they have a goal to reach (even Yamcha went from likely still being fodder to Piccolo, to being on par with Raditz in 1 year of training, and was mistaken as Goku by the Android's after 3 years of training from the Mecha Frieza mini arc). Piccolo is in a similar stance to them. King Cold is unknown but could be similar to Frieza. Then you move on to the actual relevant ones. The Saiyan's have pretty much limitless potential, and very rapid growth (which can allow them to make massive leaps even during a fight), with Zenkai boosts on top of that. Frieza could also make up for the inconceivable gap in power between his Namek Saga self and the DBS Post-Beerus Saiyans with just 4 months of training. Even if we said that Frieza was peak star level, and that base RoF Goku was bare minimum universe level (just to lower the gap as much as possible), that would still be a 88,780,641,106,222,501,571,338,780,641,106,222,501,571,338,780,641 times ******* difference. Frieza could fight base Goku in just his standard final form. Of course, I doubt Toriyama intended for that big of a jump, but it still shows how much potential Frieza has. Cell literally has that alongside the potential, growth rate, and Zenkai boosts of the Saiyans. Oh, and as for forms, it's implied (although not confirmed) that he can turn Super Saiyan, as his aura seems to resemble a Super Saiyan's, with his Super Perfect state having a SSJ2-like aura, which would mean that he can use any transformations that Frieza or the Saiyan's have.