Guardiola is into his second season in charge of City, having won three LaLiga titles – and two Champions Leagues – at the helm of Barcelona and three Bundesliga crowns with Bayern Munich.
But the Spaniard said the Premier League was not yet the most difficult, pointing to the records of English clubs in Europe.
Chelsea were the last English side to reach a decider in the Champions League, claiming the trophy in 2012, with Spain (six times), Germany (two) and Italy (two) the nations with finalists since then.