First, I have a confession to make. I didn't vote. I couldn't choose between the two top contenders. But luckily, it didn't matter - it was close, but my one vote wouldn't have made a difference.
The top two episodes were In My Time of Dying and What Is And What Should Never Be. Both feature Dean at a crossroads, forced to either accept the painful direction his life has naturally headed, or to do something that seems easuer right now, but will lead to more misery in the long run. In IMTOD, Tessa the reaper explains to Dean that he's going to die either way, but he can either follow her into whatever comes next, or refuse to leave, which seems like the easier choice, but will lead to him becoming a vengeful spirit. We don't really know what decision Dean would have made, but personally, I think he would have made the right choice - to move on. His face, when he realizes he would become the type of thing he hunts, says it all for me. Later, in WIAWSNB, Dean is given the choice of staying in the djinn's dream world and living a long happy life (knowing it's a fantasy and he's dying in the real world), or going back to his real life. And again, Dean makes the right but painful choice to leave the dream world and go back to his own brutal reality. He goes where his life naturally leads him instead of seeking what was not meant to be.
And yet, in the next episode, Dean completely abandons this conviction to do the right thing, and instead chooses to alter fate by trading his life for Sam's. The difference, of course, is that in the previous two episodes, accepting his fate doesn't leave him as the sole surviving Winchester, doesn't force him to let Sam down - in his eyes, that is. (Interestingly, when Dean is considering his options in WIAWSNB, he never says he has to go back to real life because he can't leave Sam alone. Let me point out to the Sam haters that when he is faced with a similar choice in The Man Who Knew Too Much, Sam does explicitly say that he's choosing the more dangerous option, the one that could possibly result in his death, because he can't leave his brother alone out there.)
Anyway! Enough rambling. The episode you selected as the most watchable of Supernatural S2 is:
What Is And What Should Never Be
And it's a great choice! Thanks so much to everyone who voted.