Hmmm....I don't think there is such a thing as a perfect essay. Writing is probably my best subject, and I chose to use a professional editing service because I wanted my essay to be exceptional. Essay Edge editors always offered helpful comments.
The best advice, however, came from my sister, who is a superb writer. She could shape not only the style of my essay, but also its content, in a way that an anonymous editor never could. My advice is to use a professional editing service if you think you need it and also show your essay to as many other people as will let you.
And remember that it's still going to be lame, no matter how well written it is, because application essays always come out dorky. Mine do, at least. To my mind they are more or less the 500-word equivalent of getting down on your knees and saying, please, please recognize that I am special and let me in, and by the way some grants would be nice too, if you don't mind? Everyone's opinion on just how to ask this is different, and in the end you just have to go with what sounds right to you.
Hope this helps! Lots of luck to you.
SarahL