The best I've had were all home made to a fairly similar pattern - open face on a halved baguette (ideally a mild sourdough) or similar, grated mix of extra sharp white cheddar and milder cheese (mild yellow cheddar, colby, jack, havarti, doesn't really matter since it's to smooth out the texture and add moisture to the extra sharp cheddar more than its own flavor), a very little butter and garlic on the bread under the cheese.

Have yet to see a restaurant grilled cheese come close.

A grilled-cheese-and-bacon with two sliced of sourdough or white bread and a creamy American process cheese (NOT Kraft or other cheese product) is a very enjoyable sandwich in its own right, but sans the bacon it is definitely not up to the one above.