@FormerCrimsonKing: actually 17 is slightly more powerful than 18 and later on Piccolo becomes more powerful than 17. 18 is stronger than a cell saga ssj1 and cell saga ssj 1> frieza/cooler saga ssj1 > cooler or frieza. And yes Piccolo crushes the tyrant brothers.
Piccolo should be capable of defeating both overlords with relative ease. Both Frieza and Cooler were clearly less powerful than a Super Saiyan. Piccolo at one point fought Android 17 (in an apparent even battle) who is stated as being slightly more powerful than Android 18 who in turn defeated a trained Super Saiyan Vegeta with virtually no effort. That was Piccolo before the Cell Games even began and during the Cell Games he engaged in intense training along with the rest of the Z Fighters and as a result he became significantly more powerful. Piccolo along with Vegeta and Future Trunks fought quite well against the Cell Juniors who in the dubbed anime are said to be as powerful as Perfect Cell (This is never stated in the manga) although they most likely are not as powerful as Perfect Cell. Piccolo becomes much stronger at the end of DBZ so I believe it is safe to assume that he could easily defeat both Frieza and Cooler.
I believe pre-17 Cell was supposed to be way stronger than Frieza and Piccolo is stronger than that. Plus basic Trunks beat the crap out of Frieza and Cooler. They didn't even put up a fight against him. They were just instamassacred with no real effort on Trunk's part. I believe that Trunks commented that Kamiccolo Piccolo was stronger than him at SSJ1 and I think Vegeta said something along the same lines as well.
@mavfan626: Mavfan from CV? If so good to have such a good debater from CV come over here.
Anyway Piccolo wins because since being on par with second form Freeza he has trained, then fused with Kami causing his ki to grow to a level surpassing ssj1 Vegeta, Android 19 and 20 and even being = Android 17, all well above freeza and coola i'd say.