In order to make a usable hole in the earth, there has to be a connection from the rig to the ground and a way to remove the earth as you drill. From the swivel down, this means drill pipe. But when you get near the bottom, many specialized tools are needed to successfully drill.
Starting at the bottom, obviously, the bit is the first and most important part of the bottom hole assembly (BHA). Bit selection is a complicated issue that would take more space than I have here, but most drillers have a good idea what kind of bits to run, based on overall cost per foot. A bit may drill a fast hole, but give up quickly, while another bit might last forever, but penetration leaves a lot to be desired. Compromise is necessary, based on your experience and geology. New bit designs come along, and we would be wise to keep up with the changes so we don’t get passed by our competitors. Most of the time incremental improvements help in the long run.