Relentless

adjective
1 persistent, relentless, unrelenting
never ceasing; "The relentless beat of the drums"

So yeah basically, one of the 5 campaigns we're working on is for Relentless the energy drink, and I'm currently still at the research stage. The dictionary is actually not a bad place to start. Of course I already knew what the word itself meant, however it does help to look at the meaning in its entirety, explained as well as it possibly can be, because within that dictionary entry lies several words/phrases that could greatly contribute to a strategy and a tagline.