Speaking after the game, Aguero was typically nonchalant after another deadly display of finishing.
Meanwhile, Guardiola can reflect on a week in which his side scored 15 goals without reply ahead of four games in three different competitions over the next two weeks.
But Aguero provided only half of City's scoring total.
But, with United not playing until tomorrow, City have a chance to leapfrog their local rivals.
Watford's defensive crisis has left Marco Silva scratching his head and the fans with a nasty sense of deja vu. Gabriel Jesus did well to work his way past a couple of challenges and find David Silva and the skipper's quick-thinking caught out the home defence and Gomes as his low cross found Aguero who tapped home from two yards out.
City were in exuberant mood and added a third before the break when the excellent Gabriel Jesus latched on to Aguero's cute pass to stroke home in the 38th minute.
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Both sides have made undefeated starts to the season, with City's record edging out Watford by two points, picking up one more win.
Current boss Silva said: "We analysed what happened last season and we know, for the last game, there were six centre-backs missing through injury".
Raheem Sterling was then brought down in the box by Christian Kabasele and, despite Guardiola insisting Aguero take the penalty, he hammered home to put even more gloss on a truly stunning performance.
Manchester City have made a blistering start to the latest edition of the Premier League, with their 5-0 rout of Liverpool a huge indication of just how far they have come after one season under Pep Guardiola. He deserved his standing ovation when replaced by Ilkay Gundogan, whose first appearance since December a year ago brought even more cheer to Guardiola.
Watford have played the 4-3-3 formation in three of their games before going to a 4-2-3-1 when visiting the Saints, and picking up their second win.
"Watford are just two points behind us, they made an awesome start".
If the Hornets secure all three points, they would be at the summit of English football's top flight for the first time since 1982.