By VORP, the worst SP (limiting it to guys who have only started or have started 10 or more games:
So, Towers has been the worst, but you have to give Mays a prize, too. He's done that in just 6 starts, while Towers got 12, so he's clearly been the worst pitcher per start. If you gave Mays a full year at that rate, he would cost his team almost 100 runs single-handedly.
Cabrera doesn't even deserve a nod, especially since he recently had a string of 17 IP with no runs. Chen, Lopez, Ortiz, and Loewen have all been much worse than him on the O's. Cabrera's actually in the top 150 pitchers.