Lefty Grove, and I don't think it's very close.
Grove is a good balance of peak dominance vs. career length. Koufax is a fairly short peak, for me. Grove won 9 ERA crowns, Koufax pitched 9 full seasons (and partial, non-qualifying years his first 3).
That Grove pitched in an extreme offense era hurts his counting stats, and I think this has held him back somewhat in the general consensus. His 3.06 ERA doesn't seem that impressive in and of itself for an old timer, but 9 ERA crowns and a 148 ERA+ career shows how dominant he was.
Honorable Mentions for Koufax, Spahn, Carlton, Ford, Randy Johnson, and Carl Hubbell who would probably come next on my list in some order. Without digging too deep, Spahn and Johnson would probably rank 2 and 3.
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