Good data makes a new system useful from day one. Poor data follows you into the new system and stays there.
Customers and suppliers
Merge duplicates, fill in missing tax and contact details, and archive partners you have not worked with in years.
Products
Agree on one naming convention, one unit of measure per product and a clear category structure before importing.
Opening balances
Decide the cut-over date with finance and check stock and account balances against it. Do a trial run, compare, then repeat.
Migrate what you need to run the business, not everything you have ever recorded.