Best lefty off all time? My vote is Koufax!

[Exhibitman]

I like to look at peak performance and Baseball Reference has a good short-hand look at the peak 7 year performances. Seven years is enough to weed out the anomalous streaks but short enough to discount cumulative stats due to longevity:

Grove:
JAWS Starting Pitcher (7th):
106.8 career WAR | 65.6 7yr-peak WAR | 86.2 JAWS | 6.8 WAR/162

Johnson:
JAWS Starting Pitcher (10th):
101.1 career WAR | 61.5 7yr-peak WAR | 81.3 JAWS | 5.6 WAR/162

Koufax:
JAWS Starting Pitcher (90th):
48.9 career WAR | 46.0 7yr-peak WAR | 47.4 JAWS | 4.7 WAR/162

Kershaw:
JAWS Starting Pitcher (34th):
71.9 career WAR | 49.7 7yr-peak WAR | 60.8 JAWS | 6.4 WAR/162

Hubbell:
JAWS Starting Pitcher (40th):
68.5 career WAR | 47.7 7yr-peak WAR | 58.1 JAWS | 4.8 WAR/162

I'm a huge Koufax fan and Kersh is a Dodger too, but Grove was just...better. As for Kersh and Koufax (which is a big debate in True Blue LA), gotta give it to Koufax based on what went into each season (Kersh pitches a lot less) and post-season performances.