UOM first. But I would recommend not to import. You need very few and easy to maintain manually.  Then also be sure the structure is correct. 

Workcenters and locations. 

Products.

Product Variants: Be sure to read documentation on product variants carefully.  The structure in Odoo is special.  Attributes on products, create variants if they are configured correctly.  First you set up the attributes.  Then you add the values relevant for each attribute.  Then add the attributes + values to the products.  The attributes can be configured to automatic create the Product variants. So no need to import product variants.

Bill of materials when products and product attributes are final.  Be aware how Odoo can manage Bill of materials for product variants.