In the immediate future the dark one. There are 4 incoming Gen 4 T5 raids that are weak to dark, 5 if Giratina-O gets its own turn. Tryanitar won't be immediately outclassed by anything until Darkrai as Honchkrow and Weavile are on the glassy side despite their higher DPS.
On the rock front, there won't be an immediate target for it until Gen 5 or re-reruns of the birds. Even still, Rampardos is slated to heavily out-DPS it and Rhyperior will be tankier. Tyranitar is effectively a compromise between the two.