Depends on where you are, pointwise. For example anyone can "improve" up to a 8 or 9 in Verbal in about 2 hours after reading the EK guide to verbal. So if you are scoring 6's across the board its easiest to improve in Verbal and get that up to a 9 or 10. However improving your Verbal score from a 10 up to a 14 is, in all practical purposes, an impossible thing to do unless you are just born a Verbal God.
If you are already getting 9's across the board, PS is pretty easy to improve from to a 11 or 12. It is mostly just concepts and formulas. I mean really, I could summarize the entire swath of physics knowledge covered by the MCAT in about 10 pages of notes. Compared to the massive amount of knowledge demanded from BS, the PS section is pretty easy to improve a 12 or so, its nothing that 3 weeks of time and a good review book can't fix.
Now lets say you are getting 11s in each section. Here is where BS is the easiest to improve. BS is easiest to improve for perfectionists because there is a set, finite amount of stuff that can be tested. If you really studied you could easily get to the point where you would know the answer to every single question that could be asked. It's just a lot of memorizing, but getting a 15 in BS seems feasible if you have the time. Getting a 15 in PS is harder because you can't really study for it once you get to a certain point. The really hard PS questions are just intuitive and working on with new formulas that are shown to you. That is harder to study for if you are aiming for the teens.
So yea. Verbal is easiest to improve to a 10, PS is easiest to get from a 10 to a 12/13 and BS is easiest to get from low teens to a 15 👍