Has Greece left the biggest cultural impact on the world? ?

No...................................

That's going to be relative to the amount of research you do. The more you learn about the history of Greece, the stronger case you can make. I'm sure I could build a strong case w/ the internet at my fingertips in addition to what I already know. What a luxury you have that I didn't. Surely you're not requesting we do your simple work for you.

You might like also to consider China. I would not like to have to argue out which was pre-eminent.  India might also come into the picture, as might central and western Europe in general - long after the times of ancient Greece.

No today anything Greek is just ancient History same as the Romans