Correcting the vote (somehow 1 vote got lost for WYLB, making it 3 votes before mine), I'll eliminate What you leave behind. The Pah'Wraith part just didn't do anything for me.
First vote in the next round goes to The Die is Cast - it's a good episode, a good start to a two-parter. But honestly, I think it's hard to compare the individual parts of 2-part-episodes to single part ones. They just don't stand alone, therefore don't work as well individually for me.
Actually The Die is Cast is the second part. It only comes before Improbable Cause in the list because it won that season.
Anyway, I vote for the first part, because I think the second pays off what the first sets up. Granted, it's been a while since I saw these two, the others are fresher in my memory.
The Die is Cast - 2
Improbable Cause - 1
The Visitor - 0
In the Pale Moonlight - 0
Tacking into the Wind - 1