Research shows that rock is the most commonly played, and scissors the least – so paper is the best call. If your opponent is at all experienced, though, he will know that rock is the most likely opening move by an inexperienced player (particularly if he’s male, as it’s perceived as a “strong” move: “rock is for rookies”, they say) so he’ll likely call paper – and so you should call scissors.
It’s said that if your opponent plays scissors first, you can predict the type of player he is by the angle of his scissors. For instance, if he does a wide scissor then he is aggressive and may throw lots of future rocks, but if it is a thin scissor, he is experienced and you can expect lots of deceptive throws.