Theres no way Allen is the most powerful exorcist. The generals are far stronger than him, and far stronger than any of the other exorcists. However, if you don't include generals, then maybe Allen. Krory's power is insanely broken though. I doubt any of the other exorcists could defeat him in his blood form. How do you defeat that? Maybe with water? Or by evaporating it?
Edit: About the latest chapter though: I'd say it's definately a parasite type. I imagine when she deactivates it, the blood with go back inside her body. If you want to say she's not a parasite type (even though in the manga they've actually been saying she's going to become a parasite type), then you could argue that when Allen turns his arm into a sword, it's no longer a parasite type. It's just innocence being wielding by a one-armed guy. Unfortunately we don't really have any other parasite types to compare Lenalee's new innocence too.