I'm sure that there are multiple correct answers here, depending on the particular time we're talking about, but I've seen a bunch of websites claiming with great confidence that the Romans primarily ate spelt, or emmer, or millet, or wheat. So what did the majority of the people actually eat?
The Mediterranean Triad: olives, wheat, and wine.
Commonly attributed to the ancient Greeks but the Romans did adopt this. These three ingredients would likely accompany all meals.