Which is best for you depends on how you can approach the level of liquid and if you are willing to drill through the tank to make contact with the liquid. If the tank has a sight glass, the easiest solution is to install two capacitive proximity sensors on the sight glass to detect high and low levels. It's not the most precise system, but it's consistent.
Several methods require mounting through the top or wall of a tank.
One method is the capacitive level probe, which is threaded horizontally through the wall and enters the tank, so the probe comes in contact with the liquid. Sticky or gluey liquids, however, will adhere to the probe, changing its characteristics. A better solution is to install a Teflon sensor well with a capacitive proximity sensor inserted in it. Cleaning is much easier with this setup because materials don't adhere to Teflon. Another method is to use conductive level probes.