A few favorites:

A lamb and blue cheese double-dipped French Dip from Phillipe's in Los Angeles.

The soft-shell crab hoagie I got from a little neighborhood diner by the docks in Baltimore.

The pastrami sandwich on Jewish rye at Canter's Deli on Fairfax
(...where it's fun to tease the waiter by asking for kosher "ham & cheese on a bagel"...)

A torta stuffed full of carnitas, queso fresco, avocado, shreded lettuce, and salsa verde from Alberto's drive-through in San Diego