Washington is our best tackling CB, and has been for a couple years. It's why he starts.
Shaq is our quickest corner, best man-cover guy, when healthy, but his tackling skills are very mediocre.
Alexander is our best ball-skills guy, and our overall most talented CB, and has the biggest upside. But he still is young and learning. He still commonly gives up separation early in his routes (because he needs to improve technique), but often uses his talent to recover, especially against semi-accurate or less-Than-top-shelf QBs.