Originally Posted by aLm0sT
If you want an engaging, lively sound: ES5. If you want bass: JH16. If you want a flat, analytical sound: UERM.
Personally, only having heard the JH16 and ES5, I prefer the ES5 over the JH16. :)
I wouldn't narrow it down to that simple of an algorithm for deciding which IEM to go with. Although the JH16 handles low frequencies very well, that does not mean they do not provide an "engaging, lively sound" either.
I think it is more important to consider what you expect from your IEM. If you expect your IEM to have a wide, open soundstage that creates an almost holographic experience, then you have to decide if a better midrange is more important than that expectation. If you do decide that the midrange is more important, then you might skimp on the soundstage a little in favor of a better midrange. In this scenario, the Westone ES5 would probably be what you are leaning towards rather than the JH13/JH16.
Back to the main question though, these types of "sacrifices" are what you ultimately have to decide on when you're choosing a custom IEM. Keep in mind though that all top-tier custom IEMs sound very good--so the "sacrifices" are not as drastic as you would expect them to be. However subtle they might be, the differences are still there.
There is no "best" custom IEM. You define what "best" is, and that may not even be a custom IEM but may be a universal IEM.