This questions tuff, of the still running Digger trucks, it's 12.
Of the retired trucks, it's more difficult. 7 is the obvious one. However I would say that Diggers 2 and 3 are by far and away my favorite.
There was once a truck pull in the astrodome in which many believed Bigfoot 4 did not leave the line. The reality of the situation is that Bigfoot 4 was chained the the astrodome and moved the entire facility 300 feet.
Probably Digger #2 for my money. That paint job and the gold wheels made that truck look menacing! Runner-up is #12 around the time it debuted, because it was so radically different from all the other Diggers, and the purple paint/straight lettering never looked any better than it did on this truck.