Always use left-aligned text and 1.5 line spacing.
Why this matters...
Left-aligned
Left-aligned text is the easiest to read, except for some Asian and Middle Eastern languages.
Line spacing
The space helps the reader focus on the text and creates a cleaner layout.
Line length
Long lines of text requires effort to track back to the beginning of the next line.
Fully-justified text forces each line to extend to both left and right margins by adjusting word and letter spacing. Although the resulting presentation may appear more tidy at first, these spacing variations impair readability. Some circumstances, like centered headings or a right-aligned date on a document, for example, might be appropriate, but these should be treated as exceptions.