Time ATAC's or Crank Brothers Eggbeaters seem to be the favorites for 'cross. They both feature double-sided (of four-sided) entry. They don't have complicated tension mechanisms, so they tend to function better under awful conditions than other systems.
I don't know about Speedplays. Isn't their tension system recessed in the cleat? If that is the case, then I can't imagine how they could function if they were packed with mud and sand.