I love the ab workouts in the original low impact circuit, muscle endurance, and cardio and weights. Those seem to be the ones I go for over and over. There is also a good core only pre-mix on the original boot camp disc.
Pyramid upper body has the hardest ab workout, and circuit stability abs is hard too. Boot camp abs only is fabulous!!! The DVds that has variety is Coremax and Abs Circuit.
My favorites from other DVDs are:
High reps
STS total body
Athletic training
Kick punch crunch
Turbo barre
All the one's mentioned are excellent. LIC is a killer!! But "Butts & Gutts" has 2 core workouts. One with a 5lb weight and there are planks in this one and it is about 15 minutes long with several variations for your obliques and lower abs. Also a complete stability ball core work out. These are 2 separate work outs on B&G Cathe always delivers solid core work.