At what point in a line of iambic pentameter should the greatest stress be placed?
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'.
While the rhythm of iambic pentameter stresses every second syllable, (taDA taDA taDA taDA taDA), with five feet (iambs) per line, the purpose of writing in this rhythm is that it closely mimics natual spoken English. The point is for the work to sound natual and flow smoothly, so when reading iambic pentameter, it is more important to simply follow the punctuation and pause where there are commas, periods, or other break indicators.
So instead of reading a line so that it sounds like:
But SOFT what LIGHT through YONder WINdow Breaks,
Read it so it so it sounds like:
But soft, What light through yonder window breaks?
The rhythm will be there even if you don't forcefully stress it.
The stress should fall on the second syllable of each foot, Each line consists of five feet, ten syllables total in each line. Saying it out loud, the "stresses" will be da DUM/da DUM/da DUM/da DUM/da DUM.