As the question implies, what is your favorite rudimential tool you like to use when playing, particularly fills. Mine would be using diddles in my phrasing. Either as a lead into a pattern, or thrown in the middle of a pattern to add a little texture. Diddle exercises are what I used to do in high school to strengthen my left hand during my lessons and it just somehow became a fixture in my playing 25 years later. I also like going from 16ths to sextuplets through a phrase as well to help drive the beat into the next part of the song. Kinda like the break in the middle of Far Cry by Rush.