And unfortunately, without power levels, we'll never truly knowI think Mecha-Frieza too, but it's hard to tell. Goku definitely could have beaten 19 if not for the virus, but 19 was able to hurt both Goku and Vegeta. Future Trunks wasted Mecha-Frieza without trying, and he was about the same strength as Goku was three years prior to the Androids. Three years later, after lots of training, we can assume Goku has grown stronger in that time. Now, when Vegeta goes SS, he claims to be stronger than Goku (This is just my memory, plus Vegeta always thinks he is better than Goku when he gets stronger).
19's power-absorption might give him an edge, though I think that would only work if he grabbed one of Frieza's biological parts, which is only about a quarter of his body.