Rules: Voldy doesn't have any Horucrux. Everything else (including blowing up the planet with Death star) is legal
This reminds me of the debates I used to have when I was a mere child and wondered if Superman could beat the Hulk. Years and years later, I heard two kids discussing: "If Data [from Star Trek TNG] fought the Terminator, who would win?" I smiled, knowing that kids would be kids, whatever the generation.
There's a heated discussion about this one on gamefaqs right now so I thought I'd see what some fellow writers think. Personally I think Vader stomps Voldy, everything I know (which isn't much mind) about HP magic is that their worst spells aren't even that terrible. The insta death spell is a bolt of energy which can be blocked by normal objects. Like, not even fancy magical objects, just regular old stuff.