Simplest possible way of latching the voltage peak?
I think, the type of a circuit that you are looking for is called peak detector.

The simplest peak detector consists of a diode and a capacitor.  The diode prevents the capacitor from discharging, so it retains the max voltage less the diode drop.  The diode drop is a shortcoming of a simple circuit like this.  It doesn't "see" voltages that are less than the diode drop.

(That page where this snippet came from is worth skimming through.)

An OpAmp peak detector is more precise.  It compensates for the diode voltage drop.

( source )

There is a fair amount of information about peak detectors: EEVblog video, Planet Analog article.