The question is not 'who is the best substitute user', but 'who is the scariest pokemon behind a sub', correct? In which case I'd have to say kyurem-b. obviously, it's not the easiest thing to get a sub with (because of its many weaknesses), but assuming you already have a sub up? No doubt this thing is the scariest-- you are going to lose something almost without question.
That said, if we're talking about the best sub user-- I think it has to be Garchomp. Throughout fourth and fifth gen, sub Garchomp has proven itself so effective and dominant-- always being one of its best sets, and one of the metagame's as well. Whether we're talking about the era of leftovers/bright powder sand veil, or a modern salacious berry set, no Pokemon loves its little rhydon dolls like Garchomp.
"DON'T LOOK BEHIND THIS SIGN!!"