Marshal Ney, though he failed to secure the crossroads themselves.
Yes, pleanty of books refer to him promising Blucher support if practical, unfortunately, his Quartermaster General based the marching speed of the army on daytime moves, and the army was required to move by night. The subsequent delays meant that Wellington could not bring decent forces to bear until late in the day. Wellington was partly to blame for not checking De Lancey's figures fully I guess you could say, but his promise to Blucher was based on them, he did believe it possible, and would have done it had he thought it reasonable. Wellington is known for being cautious, but when clear of what he will do he is decisive such as at Salamanca for example.
In that case we can blame both of them. Grouchy did not make "bad decsisions" he was placed in an impossible situation. Napoleon handed the Prussians at least half a day's head start, gave Grouchy had less than a third of their forces and then sent Grouchy repeatedly out of touch orders. Grouchy was no incompetent, the day after Wavre he marched on Brussels only to learn about Waterloo. He then about faced his troops and extricated them from the middle of the combined allied army to return to the french-belgian border. One can blame Napoleon even more because it was Napoleon who repeatedly handed Grouchy impossible orders and then expected him to carry them out.
As for De-Erlon's corops, most people tend to blame Ney for being to hot-headed and ignoring why it was being recalled. The fact that Napoleon had not really given Ney a decent idea of what the campaign plan was, had only called him to the army just prior to the campaign, and Ney felt he needed D-Erlon to achieve his orders doesn't seem to matter.
Napoleon lost Waterloo in the day and a bit beforehand mostly. His opening strategy of invasion was sound, his inability to adapt to changing circumstances displayed sheer incompetence and arrogance.