Roast beef and swiss, havarti, or muenster is a go-to for me. I put it on a good hearty country white bread with some mayo and a little hot paprika, then press and toast it in a pan with butter till the cheese melts. It's a simple sandwich, but delicious.
If I'm at a sub shop or something though I usually get some sort of Italian meats (ham, capicola, salami, peppenero, etc.) with provolone, lettuce, onion, salt/pepper/oregano, oil & vinegar, and if they have it some pepper relish.
Favorite novelty sandwich? The Elvis Presley. Peanut butter, banana, and bacon...don't knock it till you try it, it's delicious. It's also one of the reasons he died, so go easy on them, lol.