Best Star Trek story ever told?

I think this is an impossible question to answer. As the videos in the course were about history and how Star Trek has found a way to revisit and help you rethink controversial moments of history, I started thinking along those lines. My favourite episodes in that respect have been those that took place on the Holodeck or in an alternative reality, and considered the What if...? question. 

There are a high number of such episodes, but my favourite is the two-part episode Time's Arrow from the Next Generation. The theme is interesting for me for two main reasons. It plays with the idea of alien interference in the development of human society. The writers choose a "singularity", a so-called point of no return in human history, the period of the Second Industrial Revolution leading to all developments, but also all cataclysms of the 20th century and still having a major impact on our lives.  While I haven't read any serious considerations of a potential alien or time traveller impact on this sincularity happening, there is a wide range of publications tackling the idea of alien interference much earlier on in human history (eg. Erich von Däniken's books). As the Star Trek Next Generation series also play with this idea in the episode The Chase, exploring the idea from another perspective is even more interesting.

It is also significant if you view it from the perspective of the Prime Directive so crucial in Star Trek and for the Federation. The question I asked myself with regards to this is whether it had been possible to leave Earth intact, if it really had only been Mark Twain who discovered something strange, or if such time travel would have had an effect on our evolution even if the time travellers had erased every sign of interference as much as possible.