I'd say consistently hardest would probably be Berdych or Tsonga, depending how well they're playing but Berdych has the flatter shots. However, when on, JMDP's power (on the FH) cannot be compared to anyone. It's a rocket!
Soderling hits hard but his shots have quite a lot of spin. He's the only one of the 4 I've seen live and it was hard but the ball did have a lot of spin on it.