Turn Left is one of the more powerful episodes in all of the Russell T. Davies era, in my opinion.  Not only is it a great alternate timeline story, which as a premise is relatively rare for Doctor Who, but it's also an enormously powerful character piece for Donna Noble.

It's easy to wave this away as just some set-up for the Stolen Earth, but I would argue beyond that it is a very interesting take on fascism.  It shows how suddenly and violently a society can turn.  While this does bolster the Doctor's character a bit, making him out to be this saviour of all mankind, it does so in a very specific way.  The idea is that a bug starts messing around with history and the Doctor is killed in a new timeline. Without him a number of disasters (all based on past specific episodes) begin to cause society in UK to fall apart.  Pretty soon people are being shipped away in camps and Donna can tell something is just wrong.

Plus, she is being haunted by a ghost name Rose Tyler.  I love how they handled Rose's return in Series 4 and they do a very good job at using her as a sort of harbinger for Donna.  There's something so foreboding about Rose's appearances across Series Four that make her character very interesting.  This isn't just a fun return, Rose is on a mission, trying desperately to warn the Doctor of what the Daleks are up to.  So I found that side of the story very fun and extremely moving.  It's a great episode that holds up well on its own, even without all the context and weight of what comes before and after this story.  One of my favourites.