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StringBuilder is NOT the answer for all string concatenation scenarios; String.Join could be

Yes, if you are in a loop and adding to a string, then a StringBuilder *could* be most appropriate. However, the overhead of spinning up a StringBuilder instance makes the following pretty dumb:     

var sb = new StringBuilder(); 
sb.Append("This is ");
sb.Append("not more efficient");
sb.Append(" solution.");
var str= sb.ToString();
 Instead, use String.Join, which is typically more performant than spinning up a StringBuilder instance for a limited number of strings. It’s my go-to concat option:  

        string myString = String.Join(" ", new String[] { "This", "is", "a", "much", "better", solution, "."}); 

The first variable of " " can just be set to "" when you don’t want a delimiter.
For loops that do a lot of looping, sure, use a StringBuilder. But just don’t assume it’s the de facto solution in all, or even the majority of cases. My rule of thumb is to add strings together when I’ve got one to five of them (likewise with String.Format if it helps with legibility). For most other cases, I tend towards String.Join. Only when dealing with a loop that isn’t limited to about 10 iterations, especially one that really lets rip, I do spin up a StringBuilder.