Comment Roll your own (Score 1) 477
I voted for bash as the most useful to learn, but for a lot of tasks I often find I have built my own application specific language.
The advantage is that you can tailor it you specific requirements - but this must not be allowed to grow into a general purpose language. I have seen an attempt to create a C like interpreted language that was unbelievably slow. A few optimisations and it was 100 time faster, and using an actual compiler got another 100 times improvement !