Я бы go об этом так: (inp = ваш фрагмент df из вопроса)
import pandas as pd
inp = [{'maximum pressure':12, 'minimum pressure':21,'Date':'2013/03/12'}, {'maximum pressure':25,'minimum pressure':14,'Date':'2015/04/16'}, {'maximum pressure':27,'minimum pressure':18,'Date':'2010/09/21'}]
df = pd.DataFrame(inp)
df['difference pressure']= df['maximum pressure']-df['minimum pressure']
df.loc[df['difference pressure'] == df['difference pressure'].max()]