Fire: Given that it jumps around a lot and flickers from one state to another, pose to pose would work best. There’s not really an “end” animation to fire. It just keeps burning, baby.
Bouncing ball: Have been using and will continue to use Straight Ahead, though I just called it sketching it out. It’s nice to have a technical term to describe what I do. Bouncing balls have a start position and an end position and don’t have many accoutrements to get in the way of Straight Ahead’s frontloaded nature.
Jumping: Starting position and an ending position, movement across a 2D arc; Straight Ahead would be what I’d use.
Water Splash: the ripples would be built up one after another, and like fire, water doesn’t have an “end shape” other than placidity. Pose to pose.
Door Opening: Well you have a starting position and an ending position and a movement across a known arc, but unless it’s a top down animation of a door opening (in which case I’d use Straight Ahead) there’s 3D movement in there and I’m not sure how I would accomplish that except by step by step transforming the door to its open version, pose to pose.
Frown to Grin: Obvious start and end position, movement (stretch) across a 2D plane along a known curve: Straight Ahead would be what I’d use.