What coffees are the least acidic? I don't like coffees with an acidic/sour taste. However, when buying coffee, the only "rating" one gets is often how "strong" the coffee is, and that does not help me. Is there something else I can go on? Say, place of origin, or certain varieties that aren't acidic in taste?
Lets answer this question with some data! My company uses machine learning, data science, and sensory science to build flavor profiling and quality control tools for the craft beverage industry. 

I will use a lot of graphs, because a picture is worth a thousand words but a graph can communicate a concept, to dispel a lot of the myths about acidity in coffee. 

TL;DR: Roast level is the Primary determinant of sour and acidity in coffee, where lighter roast has more natural acid compounds left unbroken from maillard reaction and strecker degradation (non-enzymatic browning).  This is closely followed by Altitude, green processing, and finally age of the coffee.



Differences by Region



Based on 10,000+ reviews, their seems to be little indication that you can determine the sourness or acidity of a coffee simply by where its grown! 

Now, this lack of effect would disappear if we controlled for the altitude the coffee was grown at. Why? Because higher altitude plants are under more stress, forcing them to be more efficient - less leafs, less fruit, but each leaf has a higher respiratory rate, and each fruit is more potent! 



Green Processing



This graph shows the difference in distribution of Sour &amp; Acidity of washed coffee vs natural processed coffee. There is a very small difference in the skew and kurtosis, but it is statistically significant. 

Washed Coffee brewed in a Chemex is on average more acidic than its natural washed counterpart. Why? This is most likely a cognitive, not chemical, effect. Because natural processed coffees ferment with bits of remnant fruit (the coffee cherry) they retain more sugars, some of which are preserved though the roasting process. The increase in fruity esters and sugar compounds masks the acidity - so even though there's about equal amounts, you taste it less in the natural processed coffees. 



Age of the Coffee



If you really want to avoid any acidity in coffee, you could drink it stale! But I don't suggest it - stale coffee lacks the caffeine and interesting flavors that make coffee wonderful. 

This plot shows the rate of acid degradation in coffee from its first day since roast. The red line marks the 14th day from roast, when the average coffee goes stale. 



Level of Roast



This graph shows Sour and Acidity as a function of the level of roast; as roast increases, the average perception of acidity decreases (in coffee brewed in a chemex). The grey field shows the Standard Error within the fit. 

You should note, why is the error so high at the highest level of roast? These are coffees considered "French" or "Italian" or even "Spanish" roasts - you should consider these coffees burnt. The fruity and interesting acids such as malic, phosphoric, and citric have been destroyed by prolonged heat exposure in the roaster, and these coffees contain acrylamide, a carcinogen, which is not only bad for you, but also tastes terrible. 


  The levels of acrylamide, a known carcinogen, peak early in the
  roasting process (at about 120 sec).  While acrylamide is still
  detectable in fully roasted and burnt coffees, it most likely don’t
  contribute to the unpleasant, burnt flavor in over-roasted coffee. 
  Multiple studies suggest that the main chemicals that cause bad
  flavors in burnt coffee are heterocyclic aromatic amines and
  polyaromatic hydrocarbons.  These increase with roasting temperature
  and time and many compounds that fall into these categories are known
  or suspected carcinogens.




Differences by Brewing Method

An extension to this question, reviewing the differences in the level of acidity by brewing method has been answered here: Could preparation method make coffee more acidic?