Great question. I can see, and agree why most would pick the Power Puncher. However, I pick the Speed Demon, at least as you list them here.
Here is why:
Of each of the "Speed Demons" on your list, a strong debate could be made that each could have gone undefeated throughout his career. Also, it could be argued that each was undefeated during his prime.
Whitaker: 3 losses. EVERYONE knows he was ROBBED against Ramirez and Chavez. More than half polled felt he defeated De la Hoya, even while well past his prime. His loss to Trinidad was even further past his prime.
ALI: Had it not been for his exile, Neither Frazier nor Norton would have come close to defeating ALI. His lifestyle contributed to his other losses while well past his prime. During his prime, he was Untouchable.
Pep: Look at his record and see if you think many, if any, could have derailed him during his prime. Only Sandy Saddler turned the trick. But how many Sandy Saddlers do YOU know?
Calzaghe: The debate is often that Joe fought nobody. Jones and Hopkins were well past their respective primes. Still, Calzaghe never came close to defeat.
Speaking of Speed Demons, I'll add one more.
Roy Jones Jr. He was a very powerful Speed Demon during his prime at 168. Before his ill-fated move to and from the heavyweight division, it could be argued that Jones was "not defeated", (depending on how you view the "fakeout" against Montell Griffith).
Now, if you look at the Power Punchers, each of them was defeated during his prime or close to it. Liston was past his prime against ALI but he lost early in his career. None of the Speed Demons suffered such defeats.
Foreman was defeated by ALI. It could be argued that George was still developing. However, he was Heavyweight Champion of the World when he lost to the elder, post-prime ALI.
Trinidad was taken apart by Bernard Hopkins. He also lost miserably to every defensive minded fighter he ever faced. Oh, that includes his "loss" to De la Hoya. I actually like Tito better. But I'll never agree that he deserved the decision against Oscar.
Langford had his share of losses but he was more an all-around fighter than just a Power Puncher. Also, his losses were more the result of being a boy, forced to fight grown men.
I'll take the Speed Demon.