My two favorites are Pineda Taco on about 4th and Lake and Taqueria la Hacienda, just a little further down. They are both very tasty and have a variety of fillings available. Pineda doesn't always have someone who speaks fluent English, but we have always been able to get by.
Los Ocampos is delicious, but I prefer the gordita there to their tacos.
Edit- I tried to add PIneda too. They are at about 311 Lake Street, so they are almost directly across the street from each other.
Taqueria La Hacienda
334 E Lake St, Minneapolis, MN 55408
Depends on what authentic means to you I guess. I am assuming you are talking about soft corn mini-tortilla style with a meat or fish and topped with chopped onions and cilantro, correct?
The Marisceria at Midtown Global Market has killer squid tacos and fish tacos. Tacqueria Los Ocampos at any of their spots has great variety of meat choices (Lingua, Carnitas, Al Pastor, Pollo, Carne Asada etc.) you can choose for your tacos or any other meat-laden dish as well.
Tacos at any of the stands at Mercado Central are all about the same, which is to say quite good and not much different (IMO) from one stand to the next.