I know what you're thinking. Why devote an entire page to dairy when not that much of it is applied to the taco? It can't have that much of an impact on the final product can it? These may not be your actual questions but let me explain why I have dairy here on it's own. Put simply dairy is the addition to a taco that is most likely to ruin any balance you may have acheived with the other ingredients. I will explain.
Probably one of the most popular cheeses to put on a taco, cotija cheese is an aged cheese made from cows milk and has a salty milky flavor that can be compared to feta with younger cheeses, and parmesan for older cheese. The issue I have with this particular cheese is in fact one of it's major flavor components; its saltiness, as well as its pungency. I personally find this cheese to be overwhelming to both my sense of taste and smell. I find that the smell utterly takes over any other aromas so that almost all I smell is this relatively stinky cheese which puts off my appetite and the saltiness of the cheese to me offsets the balance of tacos to the extent the I end up building tacos around the cheese. Personally not my go to on a taco.
Mexican Blend Cheese
You know this, I know this, everybody knows this; this abomination found in the refridgerated section of your local grocery store in large family-sized bags containing more of this garbage than you'd probably eat in your entire life and the next. I honestly don't know why anyone buys this asides laziness honestly, about the only thing this cheese does in melt well but other than that... It's one of the blandest cheeses I've ever had, it's dry inherently due to it needing to be packaged while shredded, and frankly you could get better results if you just bought the block seperately and shredded them yourself. I'd just avoid this honestly.
Sour cream is a dairy product obtained by fermenting regular cream with certain kinds of lactic acid bacteria. The bacterial culture, which is introduced either deliberately or naturally, sours and thickens the cream. Its name comes from the production of lactic acid by bacterial fermentation, which is called souring. So my issue with this is the function that liquid dairy plays when interacting with spice. That being it nullifies and dulls spice. This is fine if you're eating a very spicy taco however I find that often times sour cream dulls the spices and flavors that go into making the meat resulting in a blander taco.
An everyday table cream that has a neutral fresh-tasting flavor, similar in consistency to crème fraiche. It's pourable, rich, silky texture creates a shiny, white lace over your dishes. It has a pure rich and creamy taste with an ever so slight tinge of buttery sweetness. I much prefer this as opposed to sour cream on a taco flavor-wise as the lactic acid taste isn't so forward though it still does run into the slight issue of neutralizing the flavore profile of the taco a bit.