Currently rewatching the show. Just finished up on "The Christmas Invasion". I've always enjoyed this special, and feel it was a great introduction to Tennant's Doctor. Granted, it's not perfect, and Jackie is as annoying as ever (not Camille Coduri's fault; she does what she can with the script), but I always enjoyed watching Tennant figuring out what kind of Doctor he was.
Can't wait to start up Series 2 proper soon. Has some of my favourite episodes.