Moe's on Tennessee St is easily one of my favorite "quickie" Mexican places to go: you can almost always get in, eat, and get back out in an hour. The staff is friendly, remembers their frequent visitors, and makes the food right in front of you.
The only major downsides to this restaurant are (1) the noise and (2) the lack of seating.
(1) It's ALWAYS noisy in there, but it's something one has to expect being as close to campus as it is.
(2) If you're there in the busiest parts of the day, it's next to impossible to find a place to sit inside. There's been a few times I've had to go eat in my car, to be honest.
Another major plus for Moe's is that they have added tofu to their list of proteins, so it's possible to get delicious vegetarian-friendly Mexican.