Like it or not, it's the bit of the brief creative's read first (debatable if some read anything else to be honest).
It's also the hardest to write. Some lucky folks are natural born proposition writers, but the majority of us have to slave over honing a finely crafted sentence.
So what do you do it you're stuck, or, most likely, the suits are screaming for the brief to get creatives going?
You could take option 1 and make sure your entire brief is littered with gold and so well written creatives just have to read it. So laden with little bombs that any could detonate in a creative brain. Which means developing the skill of making the same point in a number of different ways. This works with the species of crreative who ignores the proposition as a matter of principle.
The proposition for this was 'beautifully made cars' but the brief was stuffed with different gems that bascially said, we need to make Skoda considerers believe they won't get laughed at by others.
You could take option 2 and write a 'task based' proposition. Basically frame a juicy creative task instead- because what get's good creatives really going is having a great problem to solve. Particularly useful when we're increasingly realising we're changing how people feel, not 'saying at them'. Also useful for proper intergration and digital which is much more about 'doing' and creating systems for people to play in."Make the Compare in Compare the Market Famous"
Or if you're really stuck, use the 'liberating not'. Whatever is opposite to what the product/brand is about use that. Somehow is makes a one two three word proposition stand out and get's creatives thinking. You could say your coffee is strong., but you could also say "Not weak". You say your fast car is 'not slow', or even 'Not boring and complacent"
Or your beer can be 'Not girlie'
Your lolly can be 'not serious'
Every brief needs a well honed 'idea' for creatives to leap off, but there are dark arts to creating that leap. I won't tell if you won't.