Some of the best an workouts hit your entire core (abs, hips, lower back, and butt). They also challenge your core at different angles.
I built my workout (Core Blast) from workouts I found here and at ab-workouts-guide.com If you like it checkout Core Abs & Stretches, Autumn Abs, Mid-Section Drill, Drill Sergeant Mom, Core & Glute Kicks, or P90x: Ab Ripper X they're fun and challenging.
Personally if you really want your abs to show try throwing some cardio exercises. Cardio is the best fat burner.
Also don't forget to eat healthy. There's an old saying "Great abs are made in the kitchen." Cut back on soda, carbs, processed meats, and sugar. Focus on eating leans proteins (turkey, fish, chicken), fruits and veggies, drink plenty of water. Research the net for more ideas.
Good luck and God Bless!