Muai Thai, Krav Maga,any of the 'brutal' or kill zone arts. Its more of the person than art. A palm to the nose will kill a man, so Krav Maga has science to it. But, you get guys like Mas Oyama who are simply above and beyond the style they practice and still can kill.
Mixed Martial Artists get a time-line for fights/schedule, accept fights, and plan out drug tests. They can and do have plenty of time to run cycles. The lower the organization, the less strict they are. The UFC obviously is the most strict, but unless you are on cycle the last couple weeks...