I vote for the 645Nii but I am very biased. To me it is one of the best 645 cameras available. However, never having owned the others you mentioned I can't comment on them. The biggest weak point that people mention regarding the Pentax is the inability to change film backs in the middle of a roll. If that is very important to you then the 645Nii is not your camera. But, I have to say, I have never been worried about that issue so for me it doesn't even hit my radar screen. I do love the lenses. Since the 645D came out the lenses have gone up a bit in price so they are no longer as inexpensive as they were but they can still be had for good prices.
One thing you may want to consider though is a good TLR. You can get some awesome landscapes on 6x6 medium format with a good Rolleiflex or Yashica Mat and it is WAYYY easier to pack around than any of you other choices. Even with a meter included.
Anyway, good luck with your choice. 645 is a great format and I enjoy shooting it.