I'm limiting replies to television for two reasons. A movie has months [or years] to get the writing down and its mainly the central story. In series TV, time travel is usually just a one off episode but is not the thrust of the show.

Please list the who, what, and wheres of your show and why it is special to you.

My favorite has to be: DS9 - The Visitor. Old Jake giving up his career and family to help his dad. Best DS9 episode also.