Your standard rubber plunger with the end that pulls out is the best.
Reach in and pull out the end, this narrows down to 2" and fits into the bottom of the bowl.
Small movements are best. Moving the punger up and down, maybe an inch or two on the up and down strokes and then slightly pull up to allow the suction to pull the waste down.
Keeping some water in the bowl to seal around the rubber plunger helps.