[actinium226]

Most fusion reactors are expected to use Deuterium/Tritium as fuel both because He3 is hard to source and more importantly because the D-T "cross section" is much larger than any other reaction: https://scipython.com/blog/nuclear-fusion-cross-sections/

See how D-T peaks at ~60keV? And the peak is much higher than any of the others, including D-He3? This means you need lower temperatures to achieve D-T fusion (keV is ~temperature) and larger cross section means that you're more likely to get a fusion instead of a scatter when a D meets a T.

D-He3 is promising because it produces no neutrons and so reduces requirements for neutron shielding and handling of radioactive material (but if you're fueling a reactor with D-He3 you can't stop D-D reactions from happening, and those produce neutrons, so you still need some shielding and contamination procedures).