You just have to bring one thing? Mashed patates stay warm for a while... The way I make them when I'm trying to kill people with tastiness is for every 10lbs of patates, you put in 250ml of sour cream and one big pack of cream cheese (the philedelphia bar kind).
You can also make it the day before and reheat it if necessary.
Otherwise, how about shep's pie? That stays pretty warm for a while too.