MLB.com currently has a poll asking the subject question. The choices are Roy Halladay; John Lackey; Cliff Lee; and Javier Vasquez. As of this moment, the results are:
A. Roy Halladay 42%
D. Javier Vazquez 9%
Although Halladay is arguably the most talented pitcher on the list, he is replacing Lee, who has been almost as effective the last two years, therefore, from my perspective, Halladay’s net contribution will only result in a small incremental improvement to the Phillies.
And, while Lackey and Vasquez are both quality starters, they will essentially cancel out one another’s benefit to their respective teams while they compete head-to-head in the AL East. If one of these two can dominate their divisional opponents, then I believe they could be cosnidered to have a more significant impact.
Conversely, Lee adds a 2nd ace to the Mariners, which dramatically improves their pitching relative to the rest of the AL West and may be the ingredient to get the team back to the playoffs. To me, having the potential and ability to meaningfully increase the Mariners win total gives Lee the nod to have the most impact for his team.