Best Postseason Pitchers:
Mathewson - leads ERA over 100 innings 0.97, most shutouts 4, most complete games 10 (1912-1913: 5 games 1 Win 3 Losses only 5 Earned Runs)
Pettitte - most innings 276.2, most wins 19 (benefits from all those Yankees teams and had an ERA of 3.81)
Smoltz - 2.67 ERA over 209 innings, 15 wins, 4 saves, most k's 199, 8.6 k/9 1.144 WHIP
Schilling - 2.23 ERA over 133 innings, 2 shutouts (tied for 6th), 8.1 k/9, 0.986 WHIP
Gibson - 1.89 ERA over 81 innings, 8 CG out of 9 starts, 10.2 k/9, 0.889 WHIP
Lester - 2.55 ERA over 148 innings, 7.5 k/9, 1.027 WHIP
Ford - 2.71 ERA over 146 innings, 3 shutouts (tied for 2nd) 5.8 k/9, 1.137 WHIP
Becket - 3.07 ERA over 93.2 innings, 3 shutouts (tied for 2nd) 9.5 k/9, 0.940 WHIP
Valenzuela - 1.98 ERA over 63.2 innings, 6.2 k/9, 1.225 WHIP
Hershiser - 2.59 ERA over 132 innings, 2 shutouts (tied for 6th) 6.6 k/9, 1.106 WHIP
Bumgarner - 2.11 ERA over 102.1 innings, 3 shutouts (tied for 2nd), 7.7 k/9, 0.899 WHIP
Palmer - 2.61 ERA over 124.1 innings, 4.5 k/9, 1.214 WHIP
McNally - 2.49 ERA over 90.1 innings, 6.5 k/9, 1.096 WHIP
Koufax - 0.95 ERA over 57 innings, 9.6 k/9, 0.825 WHIP
Ruffing - 2.63 ERA over 85.2 innings, 6.4 k/9, 1.179 WHIP
Hoyt - 1.83 ERA over 83.2 innings, 5.3 k/9, 1.231 WHIP
Plank - 1.32 ERA over 54.2 innings, 5.3 k/9, 0.878 WHIP
Out of that, I'd say my top 5 is Mathewson, Koufax, Gibson, Schilling and Bumgarner
Mathewson could be the greatest pitcher of all time, but I don't know how he'd translate after the dead ball era. Gibson and Koufax had fewer starts than Schilling, but those performances were otherworldly even when compared to other pitchers of their era. Bumgarner and Schilling are basically neck and neck, but I'd give Shilling the bump for now due to where Bumgarner pitches. My personal preference is for Koufax over Gibson even though Gibson went CG almost every time out. Having an ERA under 1 is just nasty.
Koufax
Gibson
Schilling
Bumgarner
Mathewson