Difficult to say exactly which is the very sharpest but I would expect any of these would as good, if not very close to, the Samsung A900B:
Marantz 1080p projectors
Sim2 HT3000E, Nero models
Digital Projection's large DMD single chip models
ProjectionDesign's large DMD single chip models
I would expect Runco's X200i and VX3000 to be very good but there aren't any reviews or comparisons to go by. Certainly the X200i looked good at CEDIA last year.
I have read that the high end LED models aren't quite as sharp as the bulb based models although that could very much depend on which model/brand. The M150 looked very sharp when I saw it.
I have the IN83, in terms of sharpness it still compares very favourably to a lot of newer and more expensive models. However, Seegs informs me that in absolute sharpness, the Marantz's are better. He has owned just about every single chip DLP model out there so I would trust his verdict.
I would go with what Mark says as well, at a certain level it will probably depend more on the individual model than a brand per se.