The Blues booked their place in the FA Cup semi-finals by edging United at Stamford Bridge, courtesy of a volley from Ba.
Late in the match Cech made a crucial save to deny Javier Hernandez from close range and keep Chelsea ahead in the quarter-final replay.
Benitez believes that Ba's goal was the best in his time in charge at the Bridge, and also thinks that Cech's reflex save was vital.
He told The Mirror: "We have scored a lot of goals, but this was one of the best, for sure. Really important, very good technique, very difficult.
"Credit to Petr Cech. It was important. When you have a good keeper, it's healthy. It was a great save and a great goal."
Manchester City now await Chelsea in the last four at Wembley, with the tie to be played on Sunday, April 14.