I don't know why people are picking the velveeta hill to die on when that shit tastes like it had pennies boiled in it. At least defend actual cheddar or jack cheese or something that's actual, real cheese that comes in a block and not in powder or spray form.
Also cheddar or a jack is an optimal flavor cheese for grilled cheese, but you want a gruyere or a provolone for structure. Also this is going to sound America-brained, but trust me, spread mayonnaise on the bread instead of butter for a crispier exterior, a softer interior, and a little zesty bite.