There really isn't one universal makeup item that would make a huge difference in every girl. It all depends on you and what you look like. For example, if you have bad under eye circles that would be a concealer, if you're very pale that would be blush, if you're blond and have blond lashes that would be mascara, if you have real bad skin that would be foundation, etc.
In my opinion, the one thing that would make a huge difference for each and any girl would be lash extensions: linklinklink
Just to give you a few examples of what I mean. But they are quite pricey so if you want to duplicate the look (although it still won't give you the same effect) you would need to smudge black pencil liner onto your upper lash line, to give the illusion of thicker lashes and of course some mascara to lengthen, define and volumize!
No one knows I use an eyebrow pencil because it looks super natural. It really helps to make me look "neat", I don't know how to explain lol. Some people don't need them though, but my eyebrows are not very thick.