Angella and Deeney scored in quick succession to put Watford back in control and the Hornets, who were the leading goalscorers in the division last season, then turned on the style.
Lewis McGugan scored the fourth before Deeney completed his hat-trick with a powerful 20-yard strike into the corner and a last-minute penalty.
Forest snatched the points against Blackburn with a last-minute strike from Darius Henderson after a mistake from Rovers goalkeeper Jake Kean, while an own-goal from Barnsley's Scott Wiseman sealed a dramatic late victory for Blackpool.
Barnsley's defeat leaves them as one of six teams yet to win a point in the Championship and they are joined down the bottom of the table by Millwall, who were beaten 3-0 by Ipswich.
Burnley may have lost striker Charlie Austin but they had enough of a goal threat to beat Sheffield Wednesday 2-1, with Danny Ings and Sam Vokes putting them ahead before David Prutton bagged a consolation.
Derby came from behind to beat Brighton 2-1, with Chris Martin scoring both goals after Jose Ulloa had put the Seagulls in front.
Yeovil's first home match in the Championship ended in a disappointing defeat to Birmingham, who left with the points after a Danny Seaborne own goal.
QPR striker Junior Hoilett cancelled out an early strike from James Vaughan to earn a point for Harry Redknapp's side at Huddersfield.
The honours were also even at the Reebok Stadium, where Darren Pratley put Bolton ahead against the run of play before Nick Blackman equalised for the Royals from the penalty spot.