I'm not even a vegetarian, but one of my favorite sandwiches as Penn Station is their veggie sandwich. If you're not familiar with Penn Station, they mainly sell Philly Cheese Steaks, fresh cut fries, and fresh squeezed lemonade. Their Philly Cheese Steaks are fantastic, but their veggie sandwich is really awesome too. A friend of mine, who is a vegetarian, actually got me hooked on them. They're pretty simple too - it's just a hard roll, sort of like a french bread. Inside it they smear mayo, and put sauteed mushrooms and hot banana peppers, then top it with Provolone cheese, and I think a little Parmesan too, and a sprinkle of herbs, then roast the whole thing under a broiler until the cheese is all melty. It's definitely not health food, considering all the mayo and cheese on it, but it's oh so good, lol.
Another favorite of mine was Quizno's Monterey Chicken sandwich. They took it off their menu several years ago, but many of the locations would still make it for me if I asked them to. I forget what was all on it, I think it was swiss cheese and bacon maybe, along with the chicken lunch meat. But what made it really good was this roasted red pepper sauce they put on it, that was incredible. I wish they would sell something like that in the grocery store. I've tried making something like it at home but I cannot seem to get as concentrated a flavor as the sauce at Quiznos is.