I want to try to survey all the Mexican options downtown and provide a guide. The big one will be trying all the carts. For this first part, I ate at all the non-chain taquerias downtown. Full report is at the link below along with photos. Here's a summary:
El Grillo: Always named on cheap eats lists and by chefs who work downtown. It's living on reputation and the adjoining strip club. Most things are decent and a good value, but it gets too much acclaim for mediocre food.
Maya's: Maybe they don't have to be too good since they also have margaritas. But there's potential here. They make one of the better taqueria-level fish tacos, and freshly cook meats. If their stewed fillings just had more flavor they might actually be good.
El Pepes: This has one of the most tempting menus of any Mexican restaurant in Portland metro: carne en su jugo, tinga, albondigas, chile verde, mole, etc. Too bad they suck. Not only bland flavors, but they even insult the dishes by topping them with cheddar and iceberg. (See picture below.)
Mazatlan: The most typical of the taquerias. Most items are adequate, although they server one of the worst fish tacos I've had anywhere -- more breading than fish, and with the quality of the fish, that's actually a good thing. Large tacos with lots of meat. The carne asada is pretty good, though not entirely typical.