Adenosine triphosphate of course!
The end phosphate group is subject to hydrolysis, thus leaving ADP and the attachment of the phosphate group to another molecule is the energetic reaction here. It is not the hydrolysis so much, even though the last phosphate group on ATP is highly energetic and unstable, it is the attachment of this group that provides energy for such things as a conformation change in the molecule that is the target of attachment.
