Leftovers are great because they make for a quick and easy meal. I love doubling recipes and freezing 1/2 for this reason. But there are quite a few foods that freeze much better than others. For instance potatoes don't freeze well at all, and things that are heavily starched have problems too, like rice and pasta. That being said, my favorite left over dishes to freeze are soups and casseroles, which almost always contain some type of starch. Potato soup probably wouldn't hold up very well, but lasagna does because the starch is more balanced. Also, having good freezer containers really helps, as does dating the foods and proper thawing.
That said, one of my favorite leftover dishes is lasagna.
This is my favorite lasagna recipe. The only changes I make are substituting ground beef or sausage for the suggested veal (I'm just not a veal gal) and using skim milk as opposed to whole. But seriously, bechamel and lasagna, so effing good!