Such a distinction would've went to the 2009 pen, but they went from a 3.9 FIP through late June to a 4.48 FIP overall. Thank you, September and October. Here are the best relief corps in team history, as ranked by FIP, note that the two extremes came within 12 months of each other. Fun stuff:
Curious, I pulled CHONE's projected innings and FIP totals for the guaranteed relievers. From there I added 100 innings (so the bullpen would match last year's innings total) of replacement level pitching. Which is an easy way of seeing just how good the core is projected to be. Of course, this completely ignores leverage, but that's another discussion. Here's how that little experiment turned out:
So, basically the Rays need better than replacement level pitching for 100 innings and everyone to live up to their projections (or at least within the designated error bars) to have the best pen in team history. Hopefully I don't jinx them this year.