Possession and ambassador. Most of the time you can't afford to pass on the ambassador, but if you get possessed with it in your hand, there goes your gold, or your own possession, or your province...
I've only played a few games like this, but it has always worked out well for me. Here are the ones I remember playing:
This one I played just a day or two ago. I open double ambassador to my opponent's cache/courtyard. If my opponent gets an early possession, I'm dead, and his opening is probably a pretty good try at it. He picks up a potion early, but as I hoped, he can't make it to 6P due to all the junk I sent his way. He picks up a few late ambassadors to try to fight back, but it's too little, too late. Meanwhile, I expand my ambassadors so they can't be used against me and pick up two possessions. My opponent resigns before I can do any real damage with possessed ambassadors, but I get some nice cards by expanding his stuff.
http://councilroom.com/game?game_id=game-20120623-192939-4d956e61.htmlI posted this one in the "I'm sorry" thread. This game was cruel, and probably needlessly so. Possession, ambassador, king's court, scrying pool. I open double ambassador vs. his ambassador/potion. My play should (and does) produce the better engine but it's more vunerable to possession, particularly an early one. The strong engine pays off and lets me possess my opponent an obscene number of times--13 times on turn 22. In those 13 possessions, I steal a scrying pool, city, festival, gold, and king's court.
http://councilroom.com/game?game_id=game-20120525-224133-076ae953.html