Pecans
Closest match1 cup walnut halves = 1 cup pecan halvesNearly the same shape, size and oil content, and they behave identically in a batter, which makes this the most seamless nut exchange there is.
Trade-offSweeter and without the tannic edge, so a brownie made with pecans reads as milder and more buttery.
Allergen checkPecans are a tree nut and are frequently handled on the same equipment as walnuts and other nuts. Read the packet for shared-line warnings. If someone at the table has a nut allergy, this swap does not address it and the question belongs with their doctor.