The exact answer depends on your hitting controls (Timing/Zone/Analog/etc) so I'll just assume you're using Timing.
Not all of these apply with every level, for example, I don't think the pitching quality becomes any better from Hall of Fame to Legend, and I don't think it's any harder to hit a line drive on All-Star than it is on Veteran.
And I haven't stripped the game code down or waterboarded an SCEA employee or anything, so take these with a grain of salt. This is just what I've observed while changing difficulty levels over the years.