I still remember an amazing sandwich I had in Boston over ten years ago. It was just wonderful. It was on banana bread, and had pickled red cabbage, smoked ham and mango chutney. It was amazing, but I've never been able to recreate it.
The best sandwich I've made is inspired by Friends, when Ross has the Thanksgiving leftover sandwich with a 'moist maker' in the middle. I always do one now, when I have a roast chicken (or at Christmas). Soak one slice of bread in gravy, and sandwich that between two normal slices of bread and cold meat. I usually do bread, then chicken/turkey/then the gravy soaked slice of bread, then some cranberry sauce or brie (or both) or maybe some lettuce, and then the third slice of bread. Mmmm, the gravy makes it all moist and tasty. Yum.