Ugh its such a hard choice! I mean I think I have to go with Wash's death because I had actually seen Serenity before watching Firefly (don't hate me!) and I still sobbed like a baby. Plus it gave way for the next scenes where, like Whedon said, you really believe this could be one of those movies where some, or even all, of the main characters die. It brought a tone to the end of the movie that was necessary for it to be so brilliant.
Both equally shocking though.
I vote for Tara because her death also led to the Dark Willow arc and the yellow crayon speech and all that stuff.