Has to be ableton live for me.
Recently upgraded from version 5 which i owned on day of release, to version 8. On the top all appeared to me much the same, but after experimenting and watching tutorial vids I am totally blown away with how powerful and flexiable it has become.
The intrument and effects racks with there macro controls, and there abilty to be nested, with multipul chains in each rack has really opened my eyes to expermenting.
The new groove tool is getting a good work out aswell.