Which is the toughest? I don't mean which is the toughest to achieve in a career. That's obvious.
I mean, if you are Rodriguez at 499, Glavine at 299 or Bonds at 754, how hard is it to get that one extra notch?
I'd say Rodriguez has it easiest. He's the youngest. He's going to get #500 and may eventually get 755. It's just a matter of time for him. He ought to take the same approach at the plate as always. It's a great accomplishment, but it will pale in comparison to other records he will eventually achieve.
Bonds has more media scrutiny, by far, and he's also the one of this group that is least likely to have fan support for his effort. That's some serious pressure. That pressure increases in San Francisco, because he wants to hit it for the home fans. But, like Alex, Barry's going to hit at least one home run before its over. He is going to break the record. So it's tougher for him than Rodriguez.
But I'd argue Glavine has it the toughest. Of the three, his skills have declined the most. He's hanging on as a 4th starter. In addition, he has less control over the situation. He can pitch great and get no run support. His defense can let him down, etc. It is likely he'll get the win, but it is possible that he strings together a bunch of no decisions or losses. The risk of injury is also higher for a pitcher.
I predict they happen in that order: Rodriguez to 500; Bonds to 755; Glavine to 300.