This one's difficult...I like most of them. Though in the end, my vote went to miner. This one is the most versatile of the destructive skills and uses a lot of tricks. There's a lot of creative uses for the miner which I've noticed in some levels of Revenge of the Lemmings. Pieuw also has a whole rank dedicated to miners in PimoLems, although I haven't played that rank yet.
Also, what did you put the walker in there for?