What are considered the most effective (& easiest) workouts to get a flatter tummy?

Planks: Try planks. You hold yourself as if you were going to do push ups but don't go down. Hold as long as you can, then you can work on side planks to define the obliques.

Flatter abs: For flatter abdomen eat healthy & get physically active. Calories burned > calories consumed. To tighten ab's: crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge & “bicycling” exercise on back. Engage in regular aerobic exercise (i.e. running, swimming, Nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.)