Cy Young pitched 746 complete games in his career. Without a doubt, complete games pitched will be a record that has the lowest probability of being broken for the simple reason that in baseball today the value of a pitcher, (i.e.- salary and importance to winning), does not justify overuse. Although the term "overuse" is a relative term depending on the pitcher in question, the statistics have proven that pitcher velocity, location and effective movement are severely reduced when exceeding 100 pitches. With that said, professional baseball teams take great care in assembling a pitching staff that is comprised of starting, middle relief, long relief, and closing pitchers. Baseball management would not take this type of care in preserving their pitching arms if they simply wanted to get the most out of each pitcher.

In summary, Cy Young's record will never be broken because there no longer the opportunity for any modern day pitcher to complete this many games.

When it comes to the most difficult record to break, I admit I am a bit biased because Nolan Ryan is absolutely my favorite player of all time. He is the reason I first started watching major league baseball, and he even won his 300th game on my 15th birthday. However, I don’t believe that either his strikeout records or his record for number of no-hitters will be broken mainly because the game has simply changed too much. Ryan had an incredibly long career. Ryan made his major league debut in 1966, at the age of 19. He finally called it a career in 1993 at the age of 46. Because of this longevity, it is unlikely that anyone is going to pass his strikeout numbers. Pitchers simply don’t pitch for that length of time at the level of dominance he did. They also don’t put in the kind of innings that Ryan did. In 1974 he actually pitched over 330 innings. That is simply unheard of in this day and age, and the main reason why his career strikeout record will never be matched.

Great Question. Here are some batting, pitching and base-running records made by remarkable baseball players of MLB:
1. Pete Rose 4,256 Career Hits
2. Ed Walsh 1.82 Earned Run Average
3. Rickey Henderson 2,295 Runs Scored
4. Nolan Ryan 5,714 Strikeouts (Pitching)
5. Barry Bonds 756 Home Runs
6. Walter Johnson 110 Shutouts
7. Reggie Jackson 2,597 Strikeouts (Batting)
8. Barry Bonds 2,426 Walks
9. Cy Young 511 Pitching Wins (now here’s the record that is one of the most famous)
10. Ty Cobb 0.367 Batting Average (most prolific batting percentage)
11. Johnny Vander Meer 2 straight no hitters
12. The durable Cal Ripken played 2,632 straight games without letup (the Superman of Baseball).

The record set by Cy Young in pitching wins, consecutive games played Cal Ripken and the batting percentage of 0.367 made by Ty Cobb are I think the most difficult to shatter records in MLB.
http://www.youtube.com/watch?v=ns49XmuNQ4I
http://www.youtube.com/watch?v=A8Rnc1C_1QE
http://www.youtube.com/watch?v=V4y9ml7VhlY

In my opinion the record that will never be broken by a non-pitcher will be the .367 lifetime batting average of Ty Cobb in 11,429 at bats. He still hit .350 and .324 at the ages of 40 and 41. The closest hitter to that was Rogers Hornsby at .358, almost 10 points lower. Even the great Ted Williams is far behind at .344. Ty in his 11,429 at bats struck out only 357 times. He played in the dead ball era and still managed to be in the top ten in homers in most seasons, winning the title once.

In the modern era of baseball, with so much earning potential tied up in power numbers, the chances of breaking this record is practically nil. There would need to be a ball player similar to Ichiro Suzuki (whose current lifetime average is only .333), willing to give up power for contact and possessing enough speed to add extra hits by beating out those slow grounders.

What is also amazing is that Ty would have had even a higher average had he not played into his 40s.