I go on and on about it: Genesis of the Daleks.
It's just that I love the moral implications that the Doctor is forced to deal with. He's able to change the course of history, the way s of the universe to destroy a species that is the closest to pure evil as there can be. He'd be saving thousands, maybe millions of innocent lives, and saving himself somewhere way down along the line.
But it's destroying a whole species for an act they haven't even had the chance to commit yet. It's genocide. He'd be killing the daleks because they were daleks. He wouldn't be killing them because they've done bad things and it's for the safety of the universe - they hadn't had the chance to kill anyone, even. He did know what a terror they would become. But does that justify genocide?
Finally, he understands the other implications of killing an entire species. Inadvertently, the daleks have influenced some of the universe for the better. Different species and races have bonded together in alliance, out of fear of the daleks. If not for the daleks, they may have been in poverty or at war, or been eliminated by another force. What right does the Doctor have from preventing these relationships from forming? How many would die from lacking that alliance rather than being killed by the daleks?
It makes you think of the recent episodes. Dalek Sec, after evolving to incorporate human DNA, became a better creature, I like to think. He began to understand more, and move beyond killing everything that wasn't the same. He became flexible, as humans are, and was learning to be able to accept things that were different. Daleks were born out of racism, and were never able to understand that different is not always worse. In his evolution, Dalek Sec was almost reborn in a world where change was necessary, and different wasn't bad. And even the Doctor agreed that dalek Sec deserved a chance. That shows that the daleks are capable of changing, even if dalek Sec was unable to succeed because of the other daleks. But the potential is there - one day, in another time, the daleks could learn to accept other races. And if they can learn, they're not pure evil, that is, depending on how you define evil.
Also, the serial gives some insight on how the daleks were created - they aren't just a race that hates everything else because they are bad. They are the results of that mentality. They were created because of racism. Maybe daleks are rigid in their ways and unable to accept others, but they are only that way because they were made to be that way. And they were made to be that way from people that thought that they were better than others.
Sorry about the text wall. To be fair, I did warn you.
TL;DR it's a very interesting serial so go watch it.