When the shot really has to count - TRI-X. If you blow it in either direction - TRI-X is the most forgiving. If the wind is blowing - TRI-X - If you need to push a sheet or two - TRI-X. It is the universal film for the "can anything else go wrong scenerio" - If I can take 2 films - one is TRI-X and the other is either FP4 or APX100. Either of those will give you liquid mid tones when done in some kind of pyro. But you really need to practice with your materials before you take them in the field!