Pep Guardiola believes Manchester City's achievement of winning a domestic treble was tougher than winning the UEFA Champions League.

Guardiola spoke after seeing his team add the FA Cup to this season's Premier League title and Carabao Cup, thanks to a 6-0 win over Watford at Wembley Stadium on Sunday.

The man who twice lifted the Champions League trophy while in charge at Barcelona, but has so far failed to win in Europe with City, rates this achievement higher:

Significantly, Guardiola doesn't think this was his best campaign, even after City made history in style against the Hornets thanks to goals from Raheem Sterling, Gabriel Jesus, David Silva and Kevin De Bruyne:

The idea this hasn't been Guardiola's finest season is a tough sell since City became the first men's team in English football history to take home all three trophies.

They have joined Arsenal's women, who produced the incredible feat during the 2006/07 campaign, as the only English sides to capture the domestic treble, a fact Guardiola was keen to point out the day before the final:

City's achievement has added to the growing list of honours Guardiola has claimed as a manager during spells with Barca and Bayern Munich:

The awesome haul is almost as impressive as the standards of consistency City have set during a phenomenal run the last two seasons:

It's hard to imagine which season from his career Guardiola deems better than this one. Perhaps he rates winning the Premier League with 100 points, the way City's "Centurions" did last season, higher.

Or maybe Guardiola thinks Barca winning La Liga, the Copa del Rey and Champions League, the way they did in 2009, exceeds City's effort. He missed out on repeating the trick two years later when Cristiano Ronaldo headed Real Madrid and Jose Mourinho to a win in the Copa del Rey final.

How highly Champions League success is regarded will likely determine how well City's treble winners are remembered.

Their local rivals Manchester United won the league, FA Cup and Champions League in 1999. Manager Sir Alex Ferguson described winning a hat-trick of domestic titles in the same season as "impossible," per MailOnline's Peter Rutzler.

Ferguson couldn't do it in 1994, when a Dean Saunders-inspired Aston Villa upset United 3-1 in the Coca-Cola Cup final. United still finished as league champions and FA Cup winners but had narrowly missed out on history.

It was a similar story in 2009, when a Ronaldo-led United won the League Cup and the Premier League. However, losing a penalty shootout to Everton in the FA Cup semi-final again left Ferguson frustrated.

Guardiola, who would best the Scot in the showpiece game of the Champions League later that year, has done what Ferguson couldn't. Even so, BBC Sport's Matt Bullin pointed out how "prior to City's win at Wembley, England was the only country in the British Isles not to have witnessed a domestic treble from a men's side."

Bullin referenced Celtic, who won Scottish treble in 2018 and 2017 on Brendan Rodgers' watch, to outline how this achievement is far from unique.

However you view City's success, Guardiola's men deserve praise for their relentless winning since 2017. It's summed up by how the manager was seen taking Sterling to task after the final whistle had blown at Wembley:

Sterling had scored twice and only been denied a hat-trick by video assistant referee, but Guardiola made it clear there was room for improvement:

Thinking like this explains why City have been so dominant. It also explains why Guardiola's squad is likely to continue winning regularly next season.

Only the looming spectre of allegations of violating financial fair play rules could dampen City's spirits and chances. The club has been referred to UEFA's financial control group CFCB, according to BBC Sport, over claims of impropriety first reported by Der Spiegel in 2018.

City issued a statement via the club's official website on Thursday, dubbing the accusations as "entirely false."

Even so, it didn't stop Guardiola being asked questions on the subject after Sunday's historic win:

The outcome of the investigation won't be known for a while.

What Guardiola can focus on now is keeping his record-breaking squad in a position to add more history. He'll want to start in next season's UEFA Champions League, where the 2016 semi-final appearance under predecessor Manuel Pellegrini remains the club's best finish.