In order of preference:
1. Core Max. This DVD has 3 different segments, about 20 mins each. 1 segment uses no weight, and the other 2 segments use a medicine ball and/or a stability ball. You said you are looking for tough and thorough--these routines are it!!! Your core will BURN!!!
2. Like lainabaina said, Ab Circuits from the STS series. There are about 8 (?) different ab routines, and each routine is well-rounded and challenging. Lots of variety, and segments range from 15-20 mins.
3. Butts & Guts has 2 ab workouts: 1 that is part of the video as produced, and 1 that is a "bonus" stability ball workout. I LOVE both of these routines. The abs that are part of the regular B&G workout make my abs BURN every time. The bonus abs have a high fun factor for me. Plus, B&G is an awesome workout in and of itself, so this DVD is a great buy if you don't have it already.
Cathe also has an "Ab Hits" DVD, which is a compilation of different ab routines from older workouts. IMO, the abs on this DVD are not as "tough" as the workouts I mentioned above. Also, this DVD has never appealed to me b/c I already own all the complete workouts included on the DVD. If you own a lot of Cathe DVDs, that might be the case for you too.