This is for many people, the obvious choice in Puebla. La Fonda de Santa Clara do Mexican Food like nobody else in Puebla, but their specialty is, without a doubt the Poblana cuisine. This is an excellent place to try that mexican dish you've always wanted to eat, but you were afraid, because this is a safe clean restaurant. If you're feeling really adventurous, try the grasshoppers.
There are two locations downtown, the fist on is 1 1/2 blocks west of the zócalo. But the original one is a little bit far off, but still at walking distance if you don't mind walking five blocks. The address listed here, is for the original one.