I think Que Huong is one of the best Vietnamese places in town. The com tam (broken rice) dishes are awesome, and the weekend special My Quang is my favorite. The decor might not be elegant and may even be considered tacky, but the food is excellent. The Tofu Luc Lac is really awesome too. It's fried tofu with a sauce that's both slightly sweet and salty with a few sauteed sugar snap peas thrown in.
For those who require a more mainstream Viet restaurant, try Le Viet on Westheimer. It's owned by the same family, but caters to more non_viet folks.