Seeing as you are 13-15, do you really have much decision power as to which airline you will be flying? But never the less, please define "best." Do you wand best food? Best service? Best seat? Best entertainment? Best lounges? Best is just too broad imo. What exactly would it be that you are looking for?
Because you are flying economy, I do not think you will notice much difference between airlines. Be it an American or a European carrier, they are all pretty basic. My best advice though would be to aim for an airline/aircraft that will at give you a PTV. It really isn't until the upper classes where vast differences begin appearing.
My personal opinion: 1) British Airways 2) Lufthansa 3) American.
To sum it up though, because it is economy, just go for whoever is cheapest and most direct. Have fun in the windy city. Will expect a trip report and pictures of the late CGX