I like all the animations in a single sprite sheet, but as FearCode said, it's best if they are aligned correctly. That is that each animation fits into the same frame area, normally it should be some even multiple of 16 even if the animation has oblong dimensions. For example if the animation happens to be 58x60 at one frame and 54x62 in another and so on, then each frame should be aligned at 64x64 with the animation centered.