I'm LIS and did some ISD courses. I would say LIS, ISD, UXD, and ARM are probably the biggest (maybe in that order, but it's kind of close). A lot of people do two, but I also recommend that you just take any courses that you find interesting.
Oh I think they'll tell you this when you start, but if you see an elective course and you really know you want to do it, then take it because they only guarantee that an elective course is available once every two years.