So... this opinion comes from a non-Christian that has bitter antipathy towards organized political Christianity. So take it with that filter on.
I don't actually believe in a Satan. However, my idea of what a Satan's primary duty (within the context of Christianity) would be to turn as many people away from the Gospel as possible. After all, in Christianity, you can be forgiven if you do something bad (and we all screw up), but in that faith, the only unforgivable sin is practicing another religion or no religion at all.
So, what's a Satan to do?
Recording back-masked messages in Led Zepplin records didn't turn people away from Christ. Neither did violent video games. Neither did same-sex marriage. Neither did bad weather, earthquakes, recession, or any other personal tragedy. Say what you will about most Christians: they stick to what they believe in through thick and thin.
However... There seems to be one opening for our crafty Satan to pursue. What if there were Christian leaders that made Christianity look so repugnant that it made people of wavering faith lose their belief? Or what if it made the spiritual person who is exploring religion for the first time look at Christianity and turn away in horror? That, I believe, would be a significant victory for the Satan in question.
So, who do we know in real life that does things like that? Fred Phelps, Rick Warren, Pat Robertson, and Ted Haggert make up the "short list". However, I'd have to say that Fred Phelps wins hands-down for conducting a ministry that serves *only* to turn existing Christians away from Christ and deflects interested persons from wanting to know about Christ.