I've seen the Vander Meer no-hitters put forth before as the record least likely to be broken, but...for me it's not really in keeping with the spirit of the question. I think it's much better to ask which record is least likely to be matched. Someone getting 512 wins would exceed the existing mark by 0.2%. Someone throwing 3 consecutive no-hitters has to exceed the existing mark by 50%.

I think the spirit of the question is about which existing records are least likely to ever be achieved again. Based on how the game has changed, etc. Vander Meer's mark was not a function of how the game was played in 1938...not in any real sense. It was just a fluke. If you think about it that way - about which records are least likely to be matched, well Cy Young still looks pretty unassailable. Matching 511 wins is every bit as improbable as getting 512...but someone throwing 2 consecutive no-hitters suddenly feels a lot more possible. For me there's no comparison...511 wins is much much further away from likelihood than 2 no-hitters.

Last weekend I heard two broadcasters say we will never see a pitcher win 300 again That use to be the automatic HOF number.

Just focusing on records makes them targets and creates possibility.

Take Willie Mosconi in pool: ran 526 balls in a row in a straight pool exhibition match in 1954. Unbeatable record — for 65 years. A guy named Schmidt aimed at that record and worked on it for years, then broke through and set a new mark at 626. How impossible does that seem?

Baseball has many more variables than pool and make all these discussions fascinating.

If Ted Williams played 1920-1940, would he have challenged Ruth? What if Mays played in those years, or 1990-2010?

BTW, my kid brother and I had a private visit in 1960 with Frankie Frisch. He was a neighbor of our aunt and uncle in New Rochelle. I’ve been interested in the following years reading about his influence in getting some marginal candidates into the Hall of Fame. I’ve also known or interviewed Koufax, Frenchy Bordagaray, Denny McLain, Denny Lemaster, Ken McMullen, Steve Hovley, Al Hrabosky, Jim Colborn, Johnny Lindell, Frank Viola, Dave Magadan, Alex Ferguson, and Curt Gowdy. I was a community daily sports writer 1963-1972, and had a couple of phone visits with Sparky Anderson in his early Cincy years when he lived in Thousand Oaks, Calif. (Plus many close encounters in ballparks, restrooms, barbershops or businesses, including: Mickey Mantle, Mike McCormick, Bobby Thigpen, Willie Randolph, even Kareem Abdul Jabbar, Ralph Sampson, Sonny Liston, and all the LA Rams and Dallas Cowboys of the late 1960s. I took golf lessons from Loren Roberts before he entered the PGA tour and worked as a gofor later at 57 years old for Scott Van Pelt and the Golf Channel during the PGA in Seattle. I enjoyed it all. )