Just like choosing a great speaker, a great IEM is also genre agnostic.
No matter what you want to play on it, it should be played back faithfully, without the IEM embedding its own sound profile onto the tracks.
Unlike speakers though, the best IEM is not only one which is tuned well, but also one which fits the shape of your ears and ear canals best, which ensures that you can listen comfortably and with the full bass response.
At that point, you have two options:
A) Try a number of different universal IEMs until you find one which fits well
B) Go to a local acoustician, have them make ear impressions, then send those to a Custom IEM manufacturer, who will manufacture a pair custom-made to fit your ears perfectly.
If you decide to try option A, then a fantastic place to start is the
7Hz Zero:2.
If that one happens to fit well, then you could honestly stop right there as it has great tuning and practically non-existent THD.
Things do not get much better regardless of price.
If it doesn't fit well and you can't get a consistent seal (no deep bass, sound shifted to one side etc), then using different ear tips may also solve the issue.
This is once again down to your unique anatomy, so there's no way around some trial and error, but if you'd like you can search on ASR for "ear tips" or "eartips" and you'll find reports which ear tips models have helped other members so far.