Given the options that you have stated, progress with energy will produce progress on a wide variety of other problems because it's a physical and economic limiting factor for almost all activities. Creating cheaper, cleaner and/or more available energy will have an impact on human health and well-being, because it makes society more affluent and able to channel more resources into healthcare. Making progress on healthcare doesn't itself produce the same sort of spillover effects into as many other sectors, though it is undoubtedly beneficial. It also seems that many of the biggest challenges in healthcare are regulatory or policy-oriented and not necessarily technical, whereas many fundamental problems in the energy field are technical in nature.
I, personally, am a geologist and work for a seismic hazard and risk nonprofit that works with governments, insurance companies and other parties to better assess and ameliorate earthquake risk. My training is in more fundamental geologic research (not societally oriented at all) but I have veered towards earthquake hazards in my career because I find it to be more satisfying, both in terms of the societal benefit and in terms of my day to day effort and existence.
Earthquakes aren't anywhere as dangerous as cancer or heart disease or drunk driving, but I don't think I'd be nearly as good at solving any problems with those things. I deeply love the earth and think it's beautiful, and this motivates me to work hard at understanding it better and translating that understanding into useful information for people who want to make safer buildings and communities.