It depends if you want a Tank or a Sponge. Regirock is good as a Tank, but can't survive that well against special attack supereffective against it, such as a nice Surf or something similar. In contrast, Regice is a good Sponge that can actually take some physical hits (unlike Blissey), but when hit by a physical super effective attack (such as Fighting, Rock or Steel), it coiuld seriously damage it. Registeel is good, but it would be hard to decide which it would be a better tank as. With it's many resistances, it does make it appealing, but when it becomes apparent it's not resistent to common types such as Fire, Water, Electric, and of course Earthquake (a physical sweeper's joy) it's appeal does wear off a bit. I'm not that sure which one take my fancy to be honest... I might edit later once I've looked into it a bit more.
"A Vote for Justice is a Vote for MCD"