1. Ty Cobb's .366 lifetime batting average.
2. Nolan Ryan's career strikeout total of 5,714 (it will be rare to see a pitcher get to 3,000 Ks again).
3. Cy Young's 511 career wins as a pitcher.
4. Johnny Vander Meer's two straight no-hitters (a pitcher would need to pitch three straight in order to break this record).
It's no coincidence that most of the "unbreakable" records are pitching records. That's because pitching is the one part of the game that has changed most dramatically -- in ways that make it harder to even approach these numbers -- since these records were set.
One of the ways I determine just how difficult a record is to break is to look at the gap between the record and the second-best figure.
Hack Wilson's 190 RBIs is an amazing figure, but in the modern age of performance-enhancing drugs and occasional periods of ridiculous offensive numbers this figure isn't outlandish. Cy Young's 511 career wins, on the other hand, is almost 100 more than the next pitcher on the career list for victories (Walter Johnson, with 417). Greg Maddux had 355 career wins, the most ever by a pitcher who retired after 1965. The way pitching has changed in recent decades, the 250-win career has replaced the 300-win career as a benchmark for "great" pitchers.