You, sir, are a monster, for making people make this decision.
Stargazer is an absolute monster of a song, probably formative in what would become the 1980s metal scene. Blackmore is a god all over that thing, too. But Heaven & Hell is...Heaven & Hell, one of the finest metal songs on one of the finest metal albums of all time. I lean towards the latter - but it's still a hard, deep-cutting choice.
What's crazy is that if a band like, say, Iron Maiden released Stargazer today, it would still be seen as modern metal. It's so far ahead of its time. You can say the same for Heaven & Hell, but I think time caught up to that song faster (around 1983).