The absolutely best sauce I've ever had was those two sauces that many dollar taco places in Texas have. The tacos are nothing but a fried corn tortilla with chopped up chicken, cilantro, and onion.
The sauces come in two bottles: One red and one green. The red sauce is usually made of some type of whole dried red pepper that's first toasted over a high heat in a pan, then ran through a food processor with some water, salt, garlic, a touch of sugar, and sometimes cilantro.
The green sauce is usually tomatillos, water, salt, onions, and cilantro. I used to eat about 4 or 5 of those simplistic tacos just so I could drench them in the sauces. The sauces are also served cold in a squeeze bottle.