The #1 best (if you disagree, you are exercising your right to be wrong) cheese for grilled cheese sandwiches is Extra Sharp Cheddar.
BUT - I often enjoy a multi-cheese creation - using a few varieties, but always include a sharp cheddar. Gouda works well, as do several others.
“The #1 best (if you disagree, you are exercising your right to be wrong) cheese for grilled cheese sandwiches is Extra Sharp Cheddar.”
I don’t disagree.
Sharp cheddar with sourdough is best. Just make sure you use lots of butter when frying.