Just went on a trip to Puebla and Tlaxcala for Labor Day, figured I'd report in on what I found...
Fonda de Santa Clara: went to the second branch of this (the one further west, on 3 Pte near 11 Nte, on our first night there. Very solid. Are there better places to get a spread of Poblano specialties? Maybe. But it was close, it was convenient, it had very good service, and it was tasty. We got the chile en nogada, the chicken in mole poblano, the enchiladas de tres moles (if memory serves: mole poblano, pipian verde, and pipian negro), the mixiote, and the manchamanteles. While the two "classic" poblano dishes - the mole and the chile en nogada - were good, for me the two standouts were the mixiote (a sort of pork and lamb stew, served in what looks like a giant plastic bag) and the manchamanteles (a stew of pork, fruit, and a kind of salsa roja).
Las Ranas (2 Pte by Cinco de Mayo): Go here. You won't regret it. If you like tacos arabes (aka tacos al pastor or tacos del trompo), you'll love these. I'd never had them served in pan arabe (pita) before, but it makes them even more awesome. The torta de cochinita pibil is also a thing of beauty. You can pass on the frijoles charros, though.
Mercados de Sabores Poblanos: I wanted to try this food court (4 Pte and 11 Nte), since it seemed cool in the stuff I'd read about it online. It was fun, but it didn't seem quite... lively enough. We went at a time I would have thought would be bustling - Saturday afternoon around 2 - and while there were people there, it wasn't by any means full. A lot of the stalls were empty. We got some cemitas - I forget which place, but they all seemed fairly similar - and they were tasty, though I didn't make it down to Mercado del Carmen to compare with what they have there. I feel like this market was an experiment by the city gov't that didn't quite make it. Certainly doesn't match the atmosphere or food of other food courts I've seen in Mexico - the one in the Mercado de San Juan de Dios in Guadalajara springs to mind, as does the basement of the Inmaculada in Morelia.
Antojitos Tomy: I'm kicking myself for not writing down where exactly this place is, but I think it's at the corner of 5 Pte and 3 Sur. Maybe 5 Sur, though. We were wandering around looking for somewhere to eat breakfast, and this place was packed, so in we went! Very simple menu - quesadillas, chalupas, tacos, and the like - but all freshly made and delicious. My friend got the orange juice (fresh-squeezed) and I got a champurrado. We didn't try the cafe de olla, but it looked and smelled great. We walked by later in the day, and it was still full. It's just a little local with maybe five or six tables, but it's the perfect thing for a blast of local flavor.
Vittorio's: I swear, we did not intend to go to Puebla and get Italian food. But word to the wise: most places in Puebla not directly on the Zocalo seem to close pretty early. We took a nap when we got back from touristing, went out to eat around 8:30, and couldn't find a thing! So, Italian it was. Vittorio's is nothing to write home about, but it was fine if you're a desperately hungry tourist. They also serve Poblano food, which actually seemed better than the Italian stuff - it's what most of the tables around us were getting. We got a chile en nogada, which was actually better in some ways than the one at Fonda de Santa Clara; the sauce at Fonda was a little too creamy, while the one at Vittorio's let the flavor of walnuts shine through a little more.
La Pasita: I can't say I adored the sweet drinks they serve at this little bar on the north side of El Parian. But it's certainly an experience. It's tiny - maybe three tables, plus a counter - and full of posters, art, and old photos. They serve about 12 flavors of shots, from raisin to almond to guava to god-knows-what, and that's it. It started pouring just after we ducked in, so we made it through most of the menu. Some were better than others (I recommend the raisin and the coconut), but definitely unforgettable.
Nevados: I'm forgetting the full name of this place, but it's a bar with a terrace on the north side of the Zocalo, overlooking the square. We figured we'd pop in for a drink, and tried the house "nevado," which appears to be some conconction involving tequila, lime nieve, and sickly-sweet liqueurs in a variety of flavors. Don't follow our example.
Tirol: When we got to Tlaxcala, we initially planned to try the Fonda del Convento, about which I'd read good things. But as we approached it, we saw the gent who'd been walking up the hill in front of us turning away from the door. He regretfully informed us that it was already closed. "Pero es riquísimo," he said. Sigh. So we went to Tirol instead, which was a solid option. One of my friends got the salmon, which he said was undercooked - but since he's the sort who gets steaks well done, I'm not sure I believe him. Still, be warned. I got the rabbit, which was tasty, and my other friend got the mixiote, which was also good (though Fonda de Santa Clara's is better). The service was great - though as there were only about two other occupied tables in the restaurant, that may not be saying much. They had excellent live guitar music, as well.
Restaurante Yiandro's: At the south side of Plaza Xicohtencatl, this place looks like it has a boring menu. But after we ate, we wanted pulque (a local specialty in Tlaxcala, judging from the number of stands selling it we passed on the highway), and there was a sign in the window saying they had it. (Most of the actual pulquerias in central Tlaxcala were already closed - noting a theme here?) I feel like pulque is one of those things you either love or hate; though I'm in the former camp, I will admit that the texture reminds me of nothing so much as snot. And I've only tried the flavored kinds, having heard that the "natural" flavor leaves something to be desired. But I thought the pulque here was very tasty. We had the pina colada, mango, and guava flavors, and they were all clearly newly blended with the fresh fruits. Note that this place normally sells by the liter or half-liter; not feeling that ambitious, we just asked for one cup of each flavor, and they were happy to oblige.
So that was the trip. Echoing what I've read about food in Puebla and Tlaxcala elsewhere: I didn't have food that I thought was "OMG incredible," but I had quite a lot of food that was very good. And that can be enough, especially when you get it in the variety of flavors and types that you see in central Mexico. But I'm hoping to go back at some point, and try some of the restaurants that we passed that were inexplicably closing down at 8 or 9 pm on a weekend night!