I have an energy harvesting application and I want to use a capacitor to store energy. Due to the application being very low power, a capacitor of around 50uF might be enough. What is really important though, is to have the lowest discharge possible, since the charging of the capacitor might be happening over a long period. What type of capacitor is best suited for this case?
You are looking for a capacitor with a high leakage resistance. A 50uF electrolytic capacitor is inexpensive and easy to find, but has a lower leakage resistance than other types of capacitors. You haven't mentioned the required voltage. That is an important consideration.
Here is a chart of capacitor technologies vs. leakage resistance:
The words "Values of" in the caption appear to be an error. The image is from page 8 of KEMET Corporation, Introduction to Capacitor Technologies Copyright 2013
Image from: EPCI; ESA SPCD 2018 Symposium via https://epci.eu/capacitors-news-and-trends/