Ah I finally got my memory back. I believe that IV's was the infiltration of Tower of Babil, and IX I think had two it was Alexandria Castle with Vivi, Puck, Zidane and Blank in the beginning and Alexandria again to rescue Garnet. Huh that was funny. The first time it was to kidnap her and the second was to save her. X's was the assault on Bevelle. I'm guessing X-2's was entering Leblanc's place.

FF had some good infiltration moments

I would say that probably IX, VIII, and X would tie out for me for favorites. They all unveil powerful emotions in the stories. In VIII I had thought that the party had died for real, but since they were the main characters I had a few doubts.