There is no right answer to this one. Nobody knows whether Shakespeare intended the actors to rigidly follow the stress when performing in iambic pentameter (there's nothing, for example, in Hamlet's advice to the players about sticking to the stresses!) or whether he meant the verse to be a template around which the actors could improvise, rather like a ground bass to a jazz musician.
Moreover, it varies depending on the line itself and what you want it to mean. A regular line of verse would stress de-dum de-dum de-dum de-dum de-dum. Like this: "if MUsic BE the FOOD of LOVE play ON".
But within that, you could give each 'stressed' word more or less stress depending on what you wanted the line to mean. I'll put my heaviest stress in bold type from now on.
So if you stressed 'if MUsic BE the FOOD of LOVE....' you might suggest that the food of love could be something other than music.
But if you stressed 'if MUsic BE the FOOD of LOVE...' you might suggest that the relationship of music to love might not necessarily be that music is the food of love.
So the answer to your question is that the greatest of the stresses in any line could be put in several places - it depends on the actor, on the interpretation and on the line itself.
If you're keen to know more about this sort of question, you should take a look at John Barton's excellent book, 'Playing Shakespeare'.