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
- Aura Sphere / Close Combat
- Strength / Swords Dance
- Poison Jab / Flash Cannon / Swords Dance
- Dark Pulse
Hope I helped!