My favourite casserole is a pot-pie. It's family friendly, yet also sophisticated enough for entertaining. It can be made in advance, and with leftovers, and although chicken is the most popular variety of pot pie, I've made it with pork and turkey to successful results.
I have a few casseroles on my blog, www.familybites.ca, and here is a great recipe for pork pot pie with sweet potato biscuit topping:
http://mixingbowlkids.typepad.com/family_bites/2011/10/pork-pot-pie-with-sweet-potato-biscuit-topping.html