What is the highest atmospheric pressure ever found on earth?
For that, we must look for the lowest land area on our planet - Lowest Places on Earth - and the lowest present-day one is the Dead Sea, between Israel and Jordan. Its average atmospheric pressure, 1063 hectopascals, is about 5% higher than the sea-level value of 1013 hPa.
Looking in the past, we find the Messinian salinity crisis - about 5.96 to 5.33 million years ago, the Mediterranean Sea was blocked off from the Atlantic Ocean, and it became largely or completely dried up. Its lowest parts were some 3 to 5 km below sea level, and extrapolating from above sea level, its pressure was some 50% to 90% greater than sea-level pressure.
There may have been similar deep dried-up inland seas even earlier.