The most under-responded sections I've ever seen in a business plan are the first 3:

Problem
Solution
Market Size

I really harp on these because if you can't articulate these incredibly well, the rest of the plan falls flat.

For example, if you don't articulate the problem your'e solving really well, then the solution to that problem (or for that matter the rest of your plan) just doesn't matter to anyone.

You can download a bazillion business plan templates, but I'd highly recommend just nailing these 3 points. Also, since most business plans simply translate to a presentation/pitch deck anyway, you're going to live and die by your ability to communicate these three beautifully.