Well, as far as STAT's go, all of them are no better in Spd and HP. Regice and Regirock are pretty similar, it's just that their Atk/Sp. Atk and Def/Sp. Def are switched (Regice being the better in Sp. Atk and Sp. Def; Regirock vice versa). Registeel is your more balance Pokemon from these Legendary Titans.
As far as it goes, each of them, as you know have the same learn-set, so each of them works just fine. You may want to go with Registeel if you want good defenses, otherwise go with Regirock if you want a good balance of weaknesses and resistances. You may choose Regice if you have it's hidden ability, and make it learn Hail.
Diancie is a good Pokemon, and it can Mega evolve - well depending on the game you have. Diancie does have similar qualities as the Titans, since it has low Spd and HP, but great defenses and Atk. It also has a good move pool and good STAB moves.
In my opinion, I would choose Diancie if your able to mega evolve it, otherwise choose any one of the three. If you wanted me to decide a good one, I think Regirock is a good choice.
Hope I helped! :)