La Fiesta is probably the best in Manhattan...but that isn't saying much.
Oddly, I went to a place in Marysville, KS called Las Cabanas and it kicked anything in Manhattan all to hell. It was really good food. They had a great salsa recipe too. That is a big one in my book.
In order of importance:
1. Lots of cumin
2. Quality beef and chicken
3. a non-rank Enchilda sauce
in that order