El Azteca is not getting near enough play on here right now. That place is legit.
El Azteca make a great burrito. The first time I had one, I had a smile on my face for the next hour because it was so good. I think I ordered the California burrito, but can't remember. But in my response, I just considered the fast food places.
The Adobada burrito from Abelardo's is another good choice. The marinated pork is very tasty and plentiful. They throw some guac and pico in there, wrap it and call it good. The hot sauce is also very good.
Someone mentioned El Lupita Super Burrito in Cedar Rapids. That is another good place, although I hadn't been there in a long time and my last visit was disappointing. It happens. I'll have to try the Burrito Ahogado that laminak recommended. if I can remember it.
The Super Beef Burrito from Taco John's in the 1980's was great. I liked them so much I duplicated the recipe at home and even looked into buying a franchise. My drinking buddy and i would always make that late run to Taco Johns after the bars. I like them with a little super hot green sauce and a lot of the red hot sauce. Sometimes we got there too late and they had just closed. But they would let us in anyway and serve us free Super Burritos in exchange for beer. My buddy even got a little action with one of the girls that worked there.

I don't know what they've done over the years, but they aren't anywhere near what they used to be. I think it may be that they put far less beef in them now than they used to. Now they are just a late night back up when everything else is closed.
There once was a place in Cedar Rapids called Taco Kid that had a Sancho which may be part of the burrito family. Taco Kid predated the rapid growth of Mexican food in this area of the country. Unfortunately the guy retired and the place closed. They sold the equipment to a fast food Chinese place and tried to keep the recipes going, but they just couldn't make them right. I remember meeting the former owner in the Egg Roll House and it clearly saddened him they didn't do them justice.
A similar story will likely be told about Taco Time in Ames when the owner decides to retire. Although I don't get burritos from there, the hard tacos are about the best fast food tacos you can get, because of their crispy shells. The soft tacos are good too. There used to be a Taco Time in campus town and the soft taco was our regular Sunday Night meal when the dorm food service was closed. The Taco Time was on the side street behind Casey's, or in that general area.