If you want an engine offering high reliability and high power density compared to piston, what about Wankel engines?
Unfortunately Wankel engines are rather inefficient as well. Despite the staggering sums of money that Mazda have spent over the years, the fuel consumption of their rotary powered cars is not good compared with similar power piston engines. This is despite dual injection and ignition systems. I think for the time being conventional piston engines are the best choice for light aircraft. Whilst much can be done to improve their fuel efficiency, the problem (as usual for aircraft) is the small market compared to automotive uses, and also the product liability issues.