The only one I haven't seen is Revelations, and I honestly don't really care to. As for the first 8, I'm going with Deader as the worst. I've always liked Kari Wuhrer, but that movie just bored the shit out of me.
Also, count me in on the Inferno love. I quite like Hellseeker too, which sort of feels like a sequel more to Inferno than to the rest of the series in a lot of ways, both of them being stories primarily about one man's journey into madness and damnation. Then again, I really like Bloodline, so what do I know?
Here's my series ranking:
3. Hell On Earth