The core four
Why Accounts, Stock, Selling and Buying carry most of the system's mass, and the order worth reading them in.
Modules lists all 18 in ERPNext’s own grouping. Four of them carry most of the system: 157 entities in Accounts, 67 in Stock, and 16 and 19 in Selling and Buying — against single digits for modules like Support or Telephony. Hub entities shows the same fact from another angle: most of the top-referenced entities — Account, Item, Customer, Warehouse — live in one of these four.
Read them in this order
1. Accounts
Chart of Accounts, GL Entry, invoices, payments, taxes and budgets — read this first because Selling and Buying both post into it.
2. Stock
Item, Warehouse, Stock Ledger Entry — the quantity and value side that Selling and Buying both move.
3. Selling
Customer, Quotation, Sales Order — consumes Accounts and Stock on the order-to-cash side.
4. Buying
Supplier, Request for Quotation, Purchase Order — the mirror image on procure-to-pay.
Why this order, not another
The cross-module matrix counts every Link/Table field pointing from one module into another — it is the evidence for reading Accounts and Stock first: both are heavily referenced by Selling and Buying, and barely reference either back.