I'm going with Orochimaru cause Voldemort is a pretty pathetic loser of a villain. Seriously, Orochimaru is a ninja so he's keen on stealth killing and deception than would take out Old Man V before he could get a word off of his death spell. Also, most of Orochimaru's techniques are designed to kill whereas Old V has one. Also, Voldemort could probably be permanetly taken down by Orichimaru's "soul transfer" jutsu as that looks to do bad things to the souls of it's victims.

Also, Orochimaru has more henchman than you can wave a stick at. He has several bases scattered across the continent and Kabuto is actually more competent than anything Old V has produced thus far.