It can't be Kershaw, at least not until he redeems himself a couple times in the Post Season.
It can't be Koufax, yes some very great years, but longevity and late start does him in. (It's almost like saying Eric Gagne was better then Mariano Rivera)
It comes down to Lefty Grove vs Randy Johnson
World Series - 2-1 Grove (but RJ got one, and 3-0 with a 1.04 era vs a very tough 2001 Yankees lineup, Kershaw needs something like that before he can even get into the discussion)
All Star - 10-6 Johnson
Cy Youngs - 5-n/a Johnson (granted not fair to Grove, but Johnson's 5 is more than any other lefty in the history of it, and won it in both the NL & AL)
Wins - 303-301 Johnson
ERA - 3.29-3.06 Grove
K's - 4,875-2,266 Johnson (all of the above was pretty close between Johnson and Grove, but here is where RJ starts to step on the gas)
Perfect Games - 1-0 Johnson (pitched his vs the Braves who would finish 1st, Koufax pitched his against a Cubs team that finished 8th out of 10 teams)
No Hitters - 1-0 Johnson (threw his NoNo 14 years b4 his perfect game, un-hitable when he was young, un-hitable when he was older)
Birds killed - 1-0 Johnson
Made Kruk turn his helmet around and nearly shit his pants in the batters box - Johnson
Stuck out 19 in a game twice, 18 once (Neither Grove or Kershaw have reached 16 once).
Johnson pitched from 1988 to 2009.........so he did all of the above in the heart of the steroid ERA.
PS - RJ pitched until he was 45, Koufax 30.
Also ask yourself these questions if you had a big league career.
Would you want to have a great 4 year run, arm shot, career over at 30? Would you want to be great/dominant each and every season, but continue to tank in the post season over and over? Or would you want to pitch for 22 seasons until you were 45, win a WS, win 5 Cy Youngs, throw a NoNo and a Perfect game, K almost 5,000 hitters? Also, would you as a batter with the game on the line step into the box and try your luck with Lefty Grove or Randy Johnson, I'm guessing you'd rather take your chances with Grove. Ask Wade Boggs who he'd pick. Johnson was feared!
BEST EVER Randy Johnson.
https://www.youtube.com/watch?v=h2RsY9x8ftg