It's tough to rank them as I think they are all great. We typically go for longer trips now, so we have been trying to do all 3 of the full courses each time. For me, I would rank them:
1) LBV - It's kind of cool going through the resorts and the course is challenging, but not too bad. It's the shortest of the 3 full courses. The island greens are kind of nice and the large fountains in the course are as well. I also like that the club house is a little easier to get to while at the turn.
2) Palm - The Palm and Magnolia are very similar as far as club house and food, since they share them. I like the Palm better, since I felt that we went a little more into nature and saw more animals. The course is just a little longer than LBV and plays similar as far as difficulty. The sites of Poly and Contemporary in the background of some holes are amazing. The only downside is it's a little bit of a hike to the clubhouse at the turn, so you want to make sure you are all set for balls and tees and food, if possible.
3) Magnolia - This is the most famous and most difficult of the 3. It's very long, at almost 400 yards longer that the other 2 at the white tee boxes. This is the course they use the most holes for the PGA event from. (They actually mix holes from both the Magnolia and Palm for this) This is closer to the clubhouse at the turn and you do get a little bit of a view of the castle from one of the holes. One note on this, as I'm not sure when your trip is, but they are currently redoing holes 14-17 and the course is closed for while.
All in all, you can't go wrong on any of the courses. The staff is great and the courses are all beautiful. There is challenge to them all, but not overly so. A lot of water, but it tends to be in places that you can avoid. Good luck and have fun.