Bizarre question. It's Delpo and it's not close by any means.
HE BEAT BALLBASHERS, THEREFORE HE HITS THE HARDEST
Del Potro has more natural power, but his groundstrokes aren't really that much more powerful. He goes 80% most of the time. When he hits with full power, he hits about as hard as Soderling but flatter.
Berdych is a more DEVASTATING hitter than Delpo by far though.
What makes Delpo tough for those bashers is that he can keep the ball deep even against the biggest hitters, which makes it tough for them not to miss.
His ability is to absorb power not create it.