Laser Squad (1988) was among my favourites growing up, they are the precursor to UFO/XCOM (the original from 1993).

I will add Rebelstar (1986) on the ZX Spectrum, the granddaddy of them all. Technically there was Rebelstar Raiders beforehand but that version didn’t have a computer opponent. I feel that Rebelstar is where the design elements that defined the later games came together.