Sound travels fastest through warm air and conditions such as temperature inversions can help to amplify the sound of thunder (source). Conversely, during a thundersnow event the falling, and accumulated, snow can act as an acoustic suppressor (source). Originally I was thinking that the loudest thunder at ground levels would simply be at higher elevations, however the atmospheric conditions mentioned above, and others that I'm not aware of, have me wondering where the loudest thunder would actually occur.
The loudest thunder will be heard where the lightning bolt hits, and this could be about 160 dB for the strongest superbolts. Good places to observe and hear such bolts are on the Mediterranean Sea in winter or the Andean Plateau.
The loudness next to the channel of triggered lightning strikes is well correlated with the peak current and measurements indicate that this is as high as 142±3 dB for a peak lightning current of about 20 kA. Lightning superbolts have been observed with peak currents of almost 1000 kA, so we might expect their peak loudness to be 10 log(1000/20) = 17 dB louder, i.e. about 160 dB. Such superbolts are most common over the Northeast Atlantic, the Mediterranean, and the Andean plateau, but also occur elsewhere.