All three hires were mistakes.
When NU hired Solich, they were on top of the world , had the best program in the nation and could have hired anyone. They chose inexperienced and in house. Big mistake. The oppertunity cost was high --- they could have hired huge and did not. Solich was not a horrible coach... he was OK (better than either Callahan or Pelini)... but NU based upon it market position at the time could have reached for the moon... and chose not to.
When they hired Callahan... NU was no longer the top coaching destination (as they were when they hired Solich)... but NU still was a very, very good destination position to have. This time they went with experience --- which, generally speaking, is the right thing to do. BUT... you go with experienced excellence... not experienced mediocrity. Callahan was so bad with the Raiders his team mutinied against him. It was almost unfathomable that NU hired the guy. Disaster ensued.
By the time NU hired Pelini, the program was no longer among the elite and probably even a small notch below the next level... a top 15 type position or so. That is still a good market position to go after an experienced coach with documented success elsewhere. Whether it was the nature of the people involved in the search... just what, I am not sure... but they went with an inexperienced coach. Pelini has done an unacceptable job.
As to who caused the most damage... the question I think is too personal... I would rephrase it this way --- In what era --- the Solich era, the Callahan Era, or the Pelini era --- did the NU program slide the most? By putting it in eras rather than tagging it to just the HC, it reflects the contributions of the AD, the assistent coaches, the HC --- everyone in the program at the time.
So... NU slid the worst under the Callahan regime. Bo, is the first coach to move the arrow in the right direction (if but modestly and but for a time). Under Solich (the best coach of the three) the arrow pointed the wrong way (NU was becoming less competitive). That said, how could it not... he inherited a team that one 3 of 4 NC's! Under Callahan, the arrow again was the wrong way... big time. NU got much less competitive. Bo and his staff turned the arrow the right way and NU got better. It then stalled and now the arrow is, perhaps, pointing the wrong way again (or there is no arrow... no incline or decline). Under Bo, the program recovered a little (not as much as it should have) and has since stalled or turned the wrong way.