I say Blackwater Park. In know a lot of people like the "old school" stuff, which is awesome. But, the band has been evolving since the beginning, and I tend to see Blackwater Park as their Number of the Beast. You know....just like Maiden....Killers and the first album were great....but they really came into their own productions-wise, writing-wise, etc., with Number of the Beast.....
Of course, this would make Deliverance their Piece of Mind......and the annoying debate still rages amongst metal heads as to whether NOTB or Piece of Mind is the better album. I say, incidentally, Number of the Beast.....and I also say Blackwater Park....but I said that once, didn't I?