A really good Reuben is hard to beat.
Pastrami is exquisite if good quality and piled high (no mustard, please, that ruins it!).
A true Greek gyro with fresh tziziki sauce and feta cheese.
When I am somewhere as proletarian as Subway I sometimes get a double-meat BMT with everything except mustard and mayo, and their meatball sub is sometimes (but not always) pretty decent.
A bacon, lettuce, and tomato can be fantastic, too, if the lettuce is not iceberg, the tomato is fresh and ripe, and there is enough bacon. Also, a grilled variation of the BLT with cheddar cheese added and spinach substituted for lettuce is very satisfying.