In short, the “correct” word is ‘rack,’ but of course they should just be treated as spelling variations at this point. Probably the most sensible attitude would be to ignore the etymologies of rack and wrack (which, of course, is exactly what most people do) and regard them simply as spelling variants of one word. If you choose to toe the line drawn by the commentators, however, you will want to write nerve-racking, rack one’s brains, storm-wracked, and for good measure wrack and ruin. Then you will have nothing to worry about being criticized for—except, of course, for using too many clichés.