While I've tried to come up with specific roles for my relief pitchers, the intent is to give me tactical flexibility rather than limit their actual use. It's nice to have situationally useful strengths, but one should be careful not to trade off too much general ability for them. So all of my picks are made with the understanding that the pitchers be good in general, as well.
Originally, I had the next for slots listed just as relief pitcher, but I've come up with a few more situations I'd like to have the right tool for the job in. For instance, it would be nice to have a pitcher who's so good at generating strikeouts that he's your go to guy for situations such as runners on 2nd and 3rd, no outs. And it would be nice if he didn't issue too many walks, either. Being able to throw multiple innings or start is a plus, too. One man stands out above all the rest:
Wait, I've already picked Johnson. Hmmm. Let's try this again:
Selection: Billy Wagner
Originally, Wagner wasn't on my roster. He was always a bridesmaid, never a bride. But Wagner was (and probably still could be) a fantastic relief pitcher. He actually does possess the highest career strikeout rate of all time (33.2%) and a good walk rate (8.3%). And he kept popping up: Ridiculous against lefties but still strong against righties, crazy high K rate, excellent relief record, tiny ERA and FIP. Wagner is basically the relief pitcher alter ego of Randy Johnson. And I couldn't avoid him in searching for great pitching performances if I tried. I've written about him twice already, so I don't feel the need to say more.
I do need to note that he'd be used more as a LOOGY than Johnson, with Johnson getting longer relief appearances and some starts. And next week, who knows, maybe I'll decide that Johnson is my starter. Or that my entire pitching roster consists of 4 clones each of Johnson and Pedro Martinez with a Mariano Rivera, a Brandon Webb and a Phil Niekro thrown in to leaven the mix.
A quick glance through the K% and BB% columns on Fangraphs and one pitcher stands out quickly. Sergio Romo has a 31.2% K rate with only a 5.7% walk rate. That's fanastic and makes him ideal for this sort of role. Except... Romo, a right handed pitcher, has had much greater success against righties than lefties. A 2.17/2.29/54/57 (ERA/FIP/ERA-/FIP-) overall, he's had only 65 innings versus lefties (and about 120 versus right handers) in his career. For such small sample sizes, xFIP (expected FIP), which regresses the batted ball data (like home runs per fly ball) used by FIP, performs better. Romo's xFIP versus right handers is excellent 2.80, but an merely ok 3.92 versus lefties.
Of course, on a roster with Randy Johnson available, you could use Romo as a right handend strikeout pitcher. He compliments a pitcher like Johnson (or Wagner) extremely well. If one uses Johnson as a starter, the Romo/Wagner/Rivera combination is a fantastic relief triad. Be warned, though, that Romo's career has been short and he could still flame out. Mark Prior did.
Any great power pitcher
Johnson or Pedro Martinez being the leading cadidates, Lincecum, Koufax, Clemens, Curt Schilling and Johan "Met's First No-hitter" Santana all bring good combinations of control and strike out stuff. Of course, some of them are still haven't experienced the valleys of their careers yet. And, of course, they all make good choices as starters.