The Argentine has notched nine goals for the Citizens in their opening eight matches of the campaign.
Allardyce feels that the striker's predatory ability in and around the box has made him indispensable to Manuel Pellegrini's side.
The 60-year-old told the Evening Standard: "It's a tough question but I would have to plump for Sergio Aguero - because goals win you games - followed closely by Yaya Toure and Vincent Kompany because he will help you keep clean sheets.
"With Aguero, defenders think they have him marked but then suddenly, in a blur, he's moved the ball, shot and it's in the back of the net. Enner Valencia is a bit like that for us, hence the goal against Hull while Cristiano Ronaldo did something similar against Liverpool on Wednesday.
"That's what top players do, they take you by surprise. What do I tell my defenders to do? Pray, because some things, as hard as you practice, are unstoppable."
The Hammers face City in their Premier League clash later today.