Sugarcane, I believe.

Yes sugar cane is much cheaper and efficient. But you need very warm climate to grow it. Brazil has that and uses sugar cane for the most part. The USA does not so it uses what grows well in its interior , which is corn.

Sugarcane, but I think in terms of total plant weight vs yield even sugar beets beat corn

I live in the corn belt and can assure you that sugarcane does not grow here

that sugar does not grow there does not change the fact that corn is very inefficient for biofuel

Field corn can also be made into cornmeal, which we can eat. Which is exactly why we subsidize corn growers.

We don’t grow corn specifically to make ethanol, we grow corn because we can eat it if we lose all food imports. I’d much rather eat cornmeal for years than sugar beets.

Turning excess corn into ethanol happens to be better than burying it or throwing it away.

Sugarbeets would probably be the best for easy usage in non-tropical areas. I would think most sweet crops would beat corn though, corn by itself isn't that efficient because of how much energy goes into growing the stalk. But it is one of the most robust with a huge range of soils and climates which lends to its prevalence