I would probably pick DGD, although LDMUD is a nice and well polished driver too.
It boils down to what kinds of features you want moving forward. DGD puts a lot of effort into supporting persistence, with state dumps and atomic functions, so if that sounds like something you might want to use, it's a clear winner. If not, it's still a nice fast and small driver. Dworkin's philosophy has been to try to keep features that can be done in LPC out of the driver, so it can remain efficient and manageable.
LDMUD is far closer to LPMUD 3.1.2, and has a history leading back into the Amylaar driver. It has lambda closures and seems pretty solid, although there aren't many mudlibs publicly available as examples. I know BatMUD runs on it.
FluffOS is probably my last choice for the simple reason that it's currently not really maintained, and has some issues. However, there are quite a few mudlibs available out there to use as comparisons to see what you'll need to change in your own. If you do pick that one, the 2.x branch is the stable and not-really maintained version that you will likely want. The 3.x branch was spunoff to try and convert to C++ and has evolved to be incompatible in many ways.
