Here's How You Get the Chance to Be Someone Else

I alternately love or despise episodes where the characters get to be someone else for the duration, to inhabit an alternate life, lead a different path, feel different feelings. Those eps can be glorious wish fulfillment, devastating what-ifs, or alternately, a wasted hour with a reset button.

Having watched (last night) Season 3 Doctor Who's "Human Nature/Family of Blood", and having found myself all weepy at the end of it in spite of myself (partly hormones, partly… WWI will get me every time), I wanted to take a look at "be someone else" eps that worked and those that didn't.

( Brief ep discussion )

I, myself, often have the desire to be someone else, more in the "run away from my life and start up a business in small town with a handsome sheriff" kind of way than looking at an actual individual's life and wanting to take over that, so I'm predisposed towards them, however... I am not a fan of the reset button.

So the question is, what are your favorite "alternate character or timeline" eps in any fandom?