what is your BF? Many people think they will tone their stomach with ab exercises, but if your BF is (on average) 10% or higher you wont be toned, no matter how nice your abs are. I have kick ass abs, but they are hiding under a 14 mm caliper abdominal pinch lol.
I like the ball because it really works the core.
I think it is safe to say that the best exercise routine is the one you arent doing.. Switch it up. Dont waste your time by doing hundreds of reps either, crunch squeeze hold, lower slowly, repeat. Abs are like other muscles, in that if you just fire away tons of reps at them you will strengten them, but they will not get MUCH larger. I would aim to do sets of no more than 30 reps or so. Really conentrate on the eccentric motion too, dont just let your body fall.
If you are able to keep your abs tense and really control the motion for 20-30 reps, the lactic burn will be so damn high that you wont be able to do more reps. Add weights if you need to.
Again, you need a low BF to tone, so that might be an issue depending on how low you BF currently is.