Organizational Documents - all your formation docs, shares, etc..
Tax - 3-5 years of historical tax information (everything), along with a depreciation schedule of any assets you have
Finance - mostly quickbooks or similar exports here - your CPA can help a lot. Tricky parts are usually the transactions between owners (their family members) and the company (capital infusions, distributions...can get messy) Intercompany stuff is also always a mess. You set up a LP to hold the stock of your LLC? stuff like this will get beat to death.
Loans & Borrowings - anything dealing with loaned money, or personal guarantees
Contracts/Confidentiality Agreements - FOR EVERYTHING. Cell Phone contracts, cleaning lady contract (or description of the agreement). They'll cross reference this with your A/P and A/R. Anything with a signature.
Customers & Vendors - lists with detailed contact information and yearly spend/revenue
HR - Everything related to employees, contractors, consultants, etc...including all your insurance, retirement, bonus, free crap you give employees (gifts and snacks)
Corp Insurance Policies - all your policies you've had for the last 5 years, claims and dollars spent.
IP - copyrights, licenses, trademarks, non-competes, confidentiality agreements, patents, blah blah. This is exhaustive with Tech Companies, since its what they're really buying (along w/ brains. not physical assets)
Legal - Any lawsuits.
Govt Compliance - Any permits required to do work? Any interaction w/ a Govt Agency other than the IRS.
Environmental - if you own property or do anything with a hazardous substance....usually its an outside firm doing an environmental survey of your property.
Property - both real and personal property - any leases you have. any offices you own (and loans associated) All your personal property you own (desks, computers, misc) with serial numbers and VIN's. What a nightmare.
Import/Export - If you import and export stuff, they want all your documents. Schedules of shipments (all of them), customs audits, all your brokers that handle this stuff.