I'm a huge fan of Vietnamese Subs, and without a doubt, the subs at Thi Thi are the best in Calgary. A block away from the Bow River, and in the heart of Chinatown, Thi Thi is consistently delicious and the service is always great.
I'm pretty sure it's run by a husband-wife team, and it's neat to watch them work together in such a teeny tiny shop. They've got it down to a science.
Thi Thi is strictly take out, but there is a park and a lot of benches nearby where you can enjoy your sub. Try to avoid going during lunch time if you can; it's a pretty popular spot during the work week.
My tip would be to order ahead of time by phone to skip the line. My favourite sub is the veggie with everything (even the hot peppers).