The most favoured fuel for nuclear fusion reactor is deuterium-tritium.
This is used in all mainstream fusion projects such as JET and ITER.
Unfortunately tritium is very rare and it is impossible to obtain natural supplies because of the decay half-life of 12.3 years. Alternative fuels exist but these require much higher plasma temperatures and to achieve fusion in a reactor. Two alternative fuels are being proposed by new private companies: D-Helium3 and p-Boron11.
The conditions for fusion and the increased temperature needed to use the alternative fuels are discussed on the page, Essentials of Plasma Physics.