We used A Good Dusting to start with, but they are quite old school, and a bit too simplified for our tastes, fine if you want a fast game though.
We tend to use BP with the Sudan supplement, although I do have Sands of the Sudan which again are a bit old school I guess being based upon Peter Gilder's rules. We haven't got round to trying them yet, my fellow Sudan enthusiast read them through and wasn't overly won over by them, although there are some ideas worth borrowing.
I also have TMWWBK rules but we don't do much in the way of skirmishes for the Sudan so these haven't been tried either. We tried using a set based upon SAGA rules boards which was featured in WSS magazine many moons ago, but the Mahdists always won no matter what troops the British used or which tactics they tried so we dropped those.