“Loudest” and “highest peak” are not necessarily the same thing. Example: a 0.8 amplitude 100 Hz square wave sounds much louder than a 0.8 amplitude 100 Hz (as you can demonstrate yourself with the “Tone” generator: Tone - Audacity Manual)
For finding the highest peak level, you could try this plug-in: Find peak
For finding the “loudest”, you could try the “Analyze” option in this plug-in: ReplayGain plug-in
Note that this one needs to be applied to one track at a time and the track with the lowest (or most negative) ReplayGain level is the loudest.