REAL Mexican food in Philly?
Head to South Philly. My favorite taquerias are El Jarrocho (13th and Ellsworth) and Taqueria de Puebla (on 9th between Federal and Ellsworth). A lot of people like La Lupe, I had awful tacos there the first time and saw no reason to go back. Taqueria la Veracruzana is also highly regarded, it's on Washington around 9th.
For slightly more upscale Mexican, try Plaza Garibaldi, on Washington across the street (more or less) from Veracruzana. For moderately upscale (and awesome) Mexican, try Molcajete Mixto, 8th and Christian.
I love Plaza Garibaldi and Molcajete Mixto.
However, Northern Liberties-areaalso has some great Mexican fare. Las Cazuelas at 4th and Girard has some tasty chips and salsa, and the mole is excellent. It can be a bit crowded, especially on weekends, and is BYOB. I wasn't a huge fan of the loud mariachi band (especially if you're close to where they're playing), but they do add a nice atmosphere if they are pleasantly far away.
Slightly north of Girard on 5th Street is Taco Riendo, which is less upscale than LC, but just as tasty. They have some crispy chicken tacos that are to die for (the chicken isn't crispy, but the tortillas are), and their Mexican coffee is delicious (and excellent with their flan). Never had a bad meal there, and it's quite cheap... sandwiches (tortas) run you about $5, the same for 3 tacos.