The idea that most ancient thinkers seem to agree on is the grape, presumably because its product, wine, so easily leads human beings astray.
Folklore speaks about the apple, but this probably denotes the apple of paradise, otherwise called an etrog.
On one reading of the story one can suggest – following Samson Raphael Hirsch – that the actual identity of the fruit is not so important in itself, since what God was doing was testing the moral discipline of Adam and Eve, in order to ascertain whether they could be told “no” and have the moral strength to obey the command.
This reading echoes the Yiddish idea that in the Ten Commandments every time the word lo is found it is Loy mit an aleph, i.e. lamed-aleph, “Decidedly Not!”
The reference is to Psalm 100 where the text says that God created us v’lo anachnu, which can be read with a vav (“He made us and we are His”) or with an aleph (“He made us, not we ourselves” – i.e. we did not bring ourselves into existence).
Amongst the many other views there are some that remind us that in accordance with the original plan that man would be vegetarian, the forbidden fruit could have been any kind of vegetable, but most opinions think it was literally a species of fruit (fruit and not vegetable) because fruit seems to have a sensual quality that can attract the human appetite more than vegetables.
The test was whether Adam and Eve could control their sensuality, which could be understood in sexual terms. The Jewish concept is that appetite – including sexuality – is not an evil thing but most be enjoyed within a framework of control.