For the era breakdown:
19th century - surprisingly weak. Has to be someone by default, but none really belong in this conversation.
Deadball - Plank. It should have been Waddell, but Plank pitched another 1,500 innings and was very effective while Waddell was crazy and done by 32. Plank pitched effectively his entire career; even in limited innings at 41 his ERA was 45% better than the league. Plank had only one season below the league average ERA, and that year his FIP was .70 better than his ERA. Plank was a model of excellent consistency, very similar to Spahn. What holds Plank back is that he never really dominated the league; unlike Spahn who annihilated the league 2 times in his long career and was excellently consistent the rest of the time.
Waddell though has my favorite season by a lefty. In 1900, his first full season, Waddell led the National League in ERA, FIP, ERA+, K/9 and WHIP. He only pitched 208 innings, not that many back then, but he might have been the best pitcher in the league that season. He completed 16 of his 22 starts. He won 8 games and lost 13. What makes it so weird is that he was actually on a very good team. The Pirates won 79 games and lost 60 (Their Pythagorean is 81-58), finished 2nd in the NL that season. And yet the league ERA champion on the #2 team went 8-13. I do not think there has ever been a more tough luck season for a southpaw than this.
Pre-War Liveball - Grove
. To see how good Grove is, look at how good Hubbell was. Grove absolutely smokes him. As a Giants fan, Hubbell is my favorite of the two but facts are facts.
Post-War Vintage - Spahn. Carlton is not that far behind, but definitely behind I think. Ford was as good as either of them, but he loses significant value by his low inning count (low in the context of an all time great discussion, that is) and so comes in a way behind them. Too much has been made of Ford's winning % and not enough of Ford's ERA relative to context. He really deserves a better look than he has gotten thus far.
1980-2021 - Randy Johnson, by a country mile. Kershaw is very unlikely to have enough effective innings left in him to overcome Randy, even with Johnson's late bloom.