To cut right to the chase before any names are named, I want, no,
need a nemesis that will play on Batman's current role as an outcast and apparent villain. Not only that, but I need that villain to test Batman as a detective. I want to see Batman thinking and working things out the way he did all the time in the Animated Series. I want to see him in the bad guy's trap (or traps) and cleverly finding a way to escape it using his resourcefulness.
Naturally, the Riddler is the obvious choice to fill that role. I do feel like other characters could offer an interesting detectivey side to things, but Riddler covers all bases, is the most recognizable to people who aren't even die-hard fans (so he'll have craploads of hype, which is always good), and pretty much blows the competition out of the water.
Accompanying Riddler, I want another character to approach Batman's outcast and detective side just as beautifully: Catwoman. I doubt I need to elaborate more.
I'm not familiar with Black Mask (but I will be once I get some spare cash and stop by Barnes & Noble - hopefully they'll have something for me), but he seems pretty damn interesting to me, and a somewhat appropriate challenge for the Bruce Wayne side of the spectrum. Bruce is still a bit immature as far as development goes, so I think he needs something to test that side of his character, to give him an epiphany as to who he wants to be when he's not Batman - rather than just a playboy.
Although I will say that Harvey Dent was certainly a step in the right direction.

Nolan is brilliant.