I voted Halladay. At this point in time I feel that he is the best in the bigs. The problem with him is that in recent years he has broken down and found himself on the DL. There was concern 2 years ago that he was on the verge of tearing the ligament linked to Tommy John surgery. They shut him down but that is a concern.
Now if I had to take one guy to build my team around for the next five years (and this is not easy to say as a Yankee fan) it would be Beckett. Still young, relatively healthy career and a big game pitcher.
Also, I think Santana has peaked and will be on a steady decline the next few years