Agent 13

Known as Sharon Carter, Agent 13 is last on this list, because she hasn't proved herself just yet. In Captain America: The Winter Soldier she was assigned to protect Steve Rogers himself "“ she kind of failed at that, even if in the end she was the one to give the signal of the rebellion against the HYDRA agents.

Furthermore, even if she is supposed to be a top-notch agent (after all, she was assigned to protect the biggest soldier of all time), she doesn't seem too skilled with a gun (as seen by the fact that she missed her mark every time she shot at Crossbones).

Well, in the comics, she didn't miss, since she was brainwashed to kill Captain America. Will that happen in the MCU's Civil War? It isn't too likely "“ though we don't know whether she is team Iron Man or Team Captain America (with whom, apparently, she is involved in a romantic relationship).