I am having trouble deciding for Black 2.
They're both really good, but I think Lucario is ever so slightly better. You can get it earlier, it can run either physical or special easily, and its Fighting/Steel typing is great offensively and defensively. Its STAB nails a bit of Iris' team super effectively, and a ton of random Trainers throughout the game. It also gets some some very nice coverage moves.
Lucario @ anything Ability: any - Aura Sphere / Close Combat - Strength - Swords Dance - Rock Slide / Shadow Claw
Hope I helped!