First Shatabdi is the Bhopal Shatabdi followed by Lucknow Shatabdi. There is no way, without data at hand to know which is the most packed Shatabdi.
I would argue the Shatabdi with most slack is 12020 RNC-HWH Shatabdi.With an average falling short of 55 kph mark in this direction and despite running in a 130 kph zone, it has lots of slack. Bokaro - Dhanbad is slack and I had argue, for a 130 kph route, speeds can be managed more at the upcoming stations as well.