I like tons of different sandwiches, so I can't say which is best. But here are my tips to improve any sandwich.
1. Heat it. Whether it's grilled, broiled, panini style, or just has hot fillings, I think a hot sandwich beats a cold sandwich 99% of the time. This includes grilled PB&J...mmmmm.
2. Layer correctly. The correct order from top to bottom is bread, spread, soft veggie, meat, cheese, crunchy veggie, other crunchy, spread, bread. Spreads (dressing, mayo, mustard, hummus, oil, butter, etc) keep the bread from getting soggy. You want the cheese and meat (if using) together so the cheese melts into the meat. Crunchy stuff on top just has a better mouthfeel. I use to work at a national sandwich chain and this order is based on numerous studies they did to figure out what people like best. YMMV.
3. Almost any sandwich is made better by coating the outside in a mix of butter, Parmesan, and garlic powder, then lightly frying. It provides amazing crunch and flavor. Different spices can be used for different flavor palates....curry, pepper, rosemary, etc.
4. Don't be afraid to turn the bread inside out. When I have a bread that prevents me from doing #3 effectively (like a crusty Italian loaf) I'll coat the inside in that mix, fry it, then use the inside as the outside of my sandwich.
5. Use different textures to make the sandwich more interesting. When I was little my favorite sandwich was white bread with the thinnest possible layer of mayo, roast beef, and provolone. Now, that sounds positively awful. It seems like it would be so limp and would kind of just melt in my mouth. I like to chew my sandwiches. Interestingly, when I was a kid the reason I liked it so much was because it would just melt in my mouth. It is really hard to chew because it sticks to the roof of your mouth...blech.