I'm only going back as far as I can remember, since I've only been following drum corps seriously for the last few years:
2017- Carolina Crown (Family member in the pit, obviously biased toward this one.)
2016- Carolina Crown (Relentless)
2015- Santa Clara Vanguard (The Spark of Invention)
2014- Cavaliers (Immortal)
2013- Madison Scouts (Corps of Brothers)
I also have to mention the show that really got me into drum corps in the first place, which was 2006 Cavaliers (The Machine). I went to the semi-finals for the first time that year since my dad was playing in the Madison Scouts Alumni Corps, and, being only 12, I thought the Cavaliers' performance was one of the coolest things I had ever seen. I still go back and watch it on Youtube to this day.