Incredibly, there is a decent place here called Beartooth Grill, which is heavily southwest in its menu. Lots of other Mexican restaurants, but most are of the tasteless-slop-on-a-plate variety.
Slightly OT, but it involves a Mexican Restaurant…
I was at a place called the Old El Paso Cafe in Arlington and who came in and sat at an adjacent table but Michael “Heckuva Job, Brownie” Brown. He was having margaritas and dinner with his wife(?) and a couple of other couples. And yeah, he was wearing a white cotton button down. 
Anyway, I thought Old El Paso was good enough, but I haven’t found a really good mexican place in the DC area. If any DC dopers know of one, I’d sure like to know!
The usual answer to this is the inimitable Fonda San Miguel, but I’ve never been there myself. I enjoy the humbler, more Tex-Mex style offerings of Serranos, Flores, Casa Garcia, or the Texican Cafe.
Hard call around here…too many good ones. My personal favorite for sit-down Mexican is La Mexicana. For really, really good fast-food Mexican, I’ll hit Cuca’s. If I want a good Margarita with dinner, Jose’s. For tamales, I just wait until the band has another fund-raiser, and buy a dozen or so for the freezer. This is Southern California…you can’t throw a rock without hitting a decent-or-better Mexican place!
Taqueria Distrito Federal in Columbia Heights. Not a classy joint by any means, but it’s the best Mexican I’ve had in the area.
Tough call for Baltimore.
There’s a couple Mexican-type fusion places like Loco-Hombre. You can get a grilled salmon with banana salsa, stuff like that. Also, the place in Canton, Nacho Mama’s has good stuff (crabmeat quesadilla, e.g.).
There’s Holy Frijoles in Hampden, which some people like, but it’s really just run of the mill Tex-Mex.
Joy America has a Southwest flair. . .chicken with molè sauce, or guacamole made right at your table. This is an excellent restaurant, IMO, but expensive.
THEN, there’s the authentic places in Upper Fells Point (a hispanic part of town). These are the ones run by hispanics, for hispanics, with hispanic soap operas on the TV. A couple of these are real good (San Luis, e.g.). This is the kind of place where you just order the pork chop, or a steak, and it’s a pork chop, but they do their own thing to it.
There’s another place down there I’ve been to where you can get “suckling pig” or other weird things. Crazy soups I’ve never heard of.
There’s a guy there with a taco truck, where they tacos are like $1.25 and they’re awesome.
San Diego reporting in. The best? There are too many I haven’t tried. My favorite, though, is El Agave.
Martha’s, hands down. The only difficult part is finding where Martha’s husband has parked their restaurant on wheels!
Seriously, the only other Mexican food place is El Chicano’s, and ever since Martha quit cooking there to start her own business on wheels, it’s gone downhill.
Martha makes the best fish tacos!!!
Thanks! And, dive-y mexican places are the best!
In Reno, you can’t beat Bertha Miranda’s, on Mill Street two or three blocks east of downtown.
I beg to differ. The best in town is Las Cuatros Milpas (The Four Cacti) down just under the Coronado Bay Bridge across from Chicano Park. OMG, I am drooling just typing this… Several Mexicans I have known have secretly confessed to me that the food there is as good, if not better, than their mother’s. Honest
The next in line is a bit closer to home, North County, called Las Olas (The Waves) along the 101 in Encinitas, across from The Charthouse. Yummy!
Too many to figure, so I’ll give a shout-out to the Tamale Lady.
I grew up eating mexican food, in homes and in reataurants.
So if you mean authentic (what’s that, huh) mexican food, around here it would have to be “La Fortuna” in San Pablo, CA. Almost solely mexican clientel, beer for breakfast, plus posole, menudo, birria and the like. A tv is tuned to a mexican station, plus a radio in the kitchen, and a way too loud jukebox that turns on spontaneously, it sems.
I think breakfast (desayunos?) to mexicans refers mostly to when it’s eaten, rather than what it is.
Mmmm. Posole, with a meaty bone and a hunk of fat, sound’s good on a rainy Saturday morning. Along with those yummy carnita tacos made with two little corn tortillas (masas) and cilantro, etc.
See ya there tomorrow. I’ll be the fat old white guy with a grin on. About 9 or 10. We mexicans don’t go to breakfast early.
mangeorge
In Fort Myers, it’s Jalapenos. Specifically, the location in Fort Myers Proper. They have two newer locations that are good but not as good as the original spot. Every Wednesday is Jalapenos day for several of my coworkers and myself. We knew the place would be good because of all the mexicans eating there. They give you a lot of food for a good price and it’s always fresh.
Margaret’s Cantina in Chapel Hill.
Mama Ninfa’s, on Navigation Blvd. in Houston.
It was the original restaurant of the Ninfa’s chain, and when Ninfa Laurenzo sold the chain, she kept this one. If you’ve ever eaten fajitas, you can thank Mama Ninfa - she originated the dish.
Casa de Lara is my favorite here. After living in Texas and California, I got spoiled, but CdL is definitely among the best I’ve eaten.
Laredo’s (AKA Lardo’s) is a local favorite, but I don’t understand why. It’s greasy and dive-y, and not in a good way. The few times I’ve eaten there I’ve left feeling dirty and used.
San Diego again:
I’ve got to agree that Los Cuatros Milpas is really fricking good. Great tamales and tortillas. It’s a serious hole in the wall, but it feels authentic.
However, we like to go to Pokez. The atmosphere is kind of funky, and they have a lot of vegetarian/vegan options in addition to standard fare. The service isn’t that great, but everything I’ve tried there so far is good.
I’m also fond of Old Town Mexican Cafe - they have a good chicken tamale, and I love their fresh tortillas.
That’s probably true of the popular style we’ve come to know, but back in the day fajita was the meat, usually cheap steak but sometimes pork or chicken, cut across the grain, and marinated then grilled. When wrapped in a tortilla it was called a “taco”. Just about anything in a tortilla was a taco. Including Vienna Sausage*. 
I have no idea where the word comes from.
*Is this delicacy commonly available in Mexico, does anyone know?
There is a little place in Old Town San Diego, about two blocks from where Casa de Pico used to be. Wonderful dive, with a funky patio and marvelous food. I can find it, but I can’t remember what it’s called.