It's my time to rise and shine. 7 Years on this website talking about stuff I know nothing about, but today is different, today I rise and shine.
Unless you buy http://www.amazon.com/Breville-BGR82...dp/B003SIN0BC/ you're better off sticking with a pan. Cast iron is sexy and is the new shit on the interweb but unless your burner at home has insane BTUs and packs a lot of heat, you're better off with a good set of heavy'ish non stick pan like http://www.amazon.com/Calphalon-Unis...dp/B004RIY4J4/ - Just make sure you don't burn shit, keep your pan around 175° celsius for delicate bread like thin sliced soft breads, or if you have garlic bread with chunks of garlic in it that could burn if you go higher. You can go up to 230°F for big slice of french bread and stuff like that, the goal is to get a decent heat/sear on the bread to actually get the nice toasted flavor.
Now butter. You always butter the bread, not the pan, mostly because you want the tasty milk solids suspended in the butter to caramelize on your bread and be delicious. Stick to lower heat (175°C or so) if you butter your bread because it's pretty easy to go past the smoke, and because the goal with butter is more to caramelize it slowly'ish than to sear. However, if you keep the heat high enough you can actually skip the butter altogether on the bread and still have something really good, if you're into the healthy side of things.
Any cheese works, grating them is better because it lets thing melt a LOT easier. Don't overheat some of the hard cheese like gruyere or manchego because they will always have hard time melting and actually will separate into a weird greasy mess if you overheat them. Good cheese are obviously cheddar and all the go-to-basic stuff, if you want to go fancy anything goat based (goat gouda, etc) will tend to be pretty amazing.
You can add other stuff and still called it a grilled cheese, it's an american thing, but getting butt hurt over it means you have way too much free time and you should be using it to be making more grilled cheese. Technically a grilled ham and cheese starts to enter the realm of croque monsieur and my french ass doesn't get butthurt about it and still calls it a grilled cheese.
Last thing, make sure you add some acidity to that to cut all the fat/richness from the cheese. Tomato is the go to topping but I'm not a huge fan, you can have pretty decent results with anything balsamic vinegar based, and if you're not into acid you can get really good and creamy result by mashing an avocado with just salt/pepper/tiny bit of garlic and using it as a spread inside the sandwich after grilling. Just keep the cheese on one side, have random meat as a barrier, and you have a non sticky cheese free side that can get extra toppings after cooking (baby spinach, avocado, etc)
That's about it. I'm glad this website is finally talking about the things that matter in life.