I believe the best sandwich is one that surprises you. Sure there's a greatness in knowing the exact one you want, getting it, and being completely satisfied. But the Dijon Chicken ($6.49) at Halsted Street Deli comes from a completely standard lunch menu, yet it made my day.
Two things make this sandwich: the bread and the spice. The tomato focaccia, while not made in house but sourced from Turano, is incredibly light and fluffy. It also gives the sandwich a hint of sweetness. The Dijon gives you the nice spicy flavor that lingers in your mouth after eating it, without being too strong (this sandwich has the "Hot" tag next to on the menu, but don't fear, this is manageable even for a spice weenie). All of this gives the chicken exactly what it needs to be memorable rather than boring.